A substantial amount of the complexity of the transport package is due
the interface: we pass contexts to functions that do IO, and then jump
through hoops to respect those contexts. Instead, this patch changes
Transport and Codec's Close() to work like net.Close(): it interrupts
any outstanding IO. We demand the same semantics from the
io.ReadWriteCloser that we pass in, which seems like not a tall order.

Still TODO:

- [ ] rework the rpc subsystem's shutdown logic. The basic idea is we
spawn an extra goroutine to wait on context.Done() and then call close,
but the shutdown logic is intricate and I am too tired right now; this
patch introduces test regressions for now.
- [ ] Tweak the docs on how this changes the interface for e.g. NewConn

- Add a timeout during shutdown for aborts
- Make sure we are actually reading from the channel

Presumably this couldn't happen before we added the timeout during
shutdown, but now I am seeing an occasional panic here without this if.

This fixes some occasional test failures where questions were sent even
though the context is canceled before the relevant functions were
called; these are dependent on sendMessage to check the context before
actually doing the send.

...to mostly avoid allocations. There's now also an spsc, which mpsc
just adds a lock to. We should consider removing mpsc; it's behind a
lock in Conn anyway.

Before:

```
goos: linux
goarch: amd64
pkg: capnproto.org/go/capnp/v3/rpc
cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
BenchmarkStreaming-8   	       1	1236673765 ns/op
PASS
ok  	capnproto.org/go/capnp/v3/rpc	1.530s

(pprof) [isd@box rpc]$ go tool pprof before.cpu
File: rpc.test
Type: cpu
Time: Nov 30, 2022 at 12:51am (EST)
Duration: 1.50s, Total samples = 2.34s (155.85%)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 990ms, 42.31% of 2340ms total
Dropped 65 nodes (cum <= 11.70ms)
Showing top 10 nodes out of 173
      flat  flat%   sum%        cum   cum%
     290ms 12.39% 12.39%      540ms 23.08%  runtime.scanobject
     130ms  5.56% 17.95%      450ms 19.23%  runtime.selectgo
     110ms  4.70% 22.65%      240ms 10.26%  runtime.lock2
     100ms  4.27% 26.92%      100ms  4.27%  runtime.unlock2
      70ms  2.99% 29.91%       70ms  2.99%  runtime.casgstatus
      70ms  2.99% 32.91%       90ms  3.85%  runtime.findObject
      60ms  2.56% 35.47%       60ms  2.56%  runtime.markBits.isMarked (inline)
      60ms  2.56% 38.03%       60ms  2.56%  runtime.pageIndexOf (inline)
      50ms  2.14% 40.17%       50ms  2.14%  runtime.(*waitq).dequeue (inline)
      50ms  2.14% 42.31%      180ms  7.69%  runtime.mallocgc

(pprof) [isd@box rpc]$ go tool pprof before.mem
File: rpc.test
Type: alloc_space
Time: Nov 30, 2022 at 12:51am (EST)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 237.04MB, 62.04% of 382.06MB total
Dropped 42 nodes (cum <= 1.91MB)
Showing top 10 nodes out of 66
      flat  flat%   sum%        cum   cum%
   43.01MB 11.26% 11.26%    51.51MB 13.48%  capnproto.org/go/capnp/v3.NewPromise (inline)
   39.50MB 10.34% 21.60%    48.51MB 12.70%  capnproto.org/go/capnp/v3.(*Decoder).Decode
      31MB  8.11% 29.71%       31MB  8.11%  capnproto.org/go/capnp/v3/internal/chanmutex.NewLocked (inline)
   25.50MB  6.68% 36.39%    25.50MB  6.68%  capnproto.org/go/capnp/v3.NewMessage
   19.01MB  4.97% 41.36%    19.01MB  4.97%  capnproto.org/go/capnp/v3.(*Metadata).Put (inline)
      19MB  4.97% 46.34%   104.01MB 27.22%  capnproto.org/go/capnp/v3/rpc.(*Conn).handleCall
   16.50MB  4.32% 50.65%       42MB 10.99%  capnproto.org/go/capnp/v3/rpc/transport.(*transport).NewMessage
   15.50MB  4.06% 54.71%    15.50MB  4.06%  capnproto.org/go/capnp/v3/server.newAnswerQueue (inline)
   14.50MB  3.80% 58.51%    19.50MB  5.10%  capnproto.org/go/capnp/v3.ImmediateAnswer
   13.50MB  3.53% 62.04%    58.51MB 15.31%  capnproto.org/go/capnp/v3/rpc.(*Conn).newQuestion
```

After:

```
goos: linux
goarch: amd64
pkg: capnproto.org/go/capnp/v3/rpc
cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
BenchmarkStreaming-8   	       1	1185679591 ns/op
PASS
ok  	capnproto.org/go/capnp/v3/rpc	1.448s

[isd@box rpc]$ go tool pprof after.cpu
File: rpc.test
Type: cpu
Time: Nov 30, 2022 at 1:42am (EST)
Duration: 1.40s, Total samples = 2.22s (158.06%)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 1050ms, 47.30% of 2220ms total
Dropped 81 nodes (cum <= 11.10ms)
Showing top 10 nodes out of 166
      flat  flat%   sum%        cum   cum%
     230ms 10.36% 10.36%      510ms 22.97%  runtime.scanobject
     170ms  7.66% 18.02%      440ms 19.82%  runtime.selectgo
     140ms  6.31% 24.32%      270ms 12.16%  runtime.lock2
     130ms  5.86% 30.18%      150ms  6.76%  runtime.findObject
     100ms  4.50% 34.68%      220ms  9.91%  runtime.mallocgc
      70ms  3.15% 37.84%       70ms  3.15%  runtime.unlock2
      60ms  2.70% 40.54%       60ms  2.70%  runtime.pageIndexOf (inline)
      50ms  2.25% 42.79%       50ms  2.25%  capnproto.org/go/capnp/v3.(*Segment).writePtr
      50ms  2.25% 45.05%       60ms  2.70%  runtime.casgstatus
      50ms  2.25% 47.30%       50ms  2.25%  runtime.futex

[isd@box rpc]$ go tool pprof after.mem
File: rpc.test
Type: alloc_space
Time: Nov 30, 2022 at 1:42am (EST)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 250.04MB, 66.66% of 375.12MB total
Dropped 62 nodes (cum <= 1.88MB)
Showing top 10 nodes out of 54
      flat  flat%   sum%        cum   cum%
   55.51MB 14.80% 14.80%    63.51MB 16.93%  capnproto.org/go/capnp/v3.NewPromise (inline)
   48.01MB 12.80% 27.60%    55.51MB 14.80%  capnproto.org/go/capnp/v3.(*Decoder).Decode
      29MB  7.73% 35.33%       29MB  7.73%  capnproto.org/go/capnp/v3.NewMessage
   22.01MB  5.87% 41.19%    22.01MB  5.87%  capnproto.org/go/capnp/v3.(*Metadata).Put
      19MB  5.07% 46.26%       48MB 12.80%  capnproto.org/go/capnp/v3/rpc/transport.(*transport).NewMessage
   17.50MB  4.67% 50.93%   103.51MB 27.60%  capnproto.org/go/capnp/v3/rpc.(*Conn).handleCall
   15.50MB  4.13% 55.06%    21.50MB  5.73%  capnproto.org/go/capnp/v3.ImmediateAnswer (inline)
      15MB  4.00% 59.06%       15MB  4.00%  capnproto.org/go/capnp/v3/server.newAnswerQueue (inline)
   14.50MB  3.87% 62.92%    36.01MB  9.60%  capnproto.org/go/capnp/v3/server.(*answerQueue).fulfill
      14MB  3.73% 66.66%       29MB  7.73%  capnproto.org/go/capnp/v3/server.(*Server).start
```