This simplifies the rpc implementation by removing the monitor/coordinate
goroutine.  Table state is now protected by a conn-level mutex.  Many false
abstractions removed, since most everything needs to be able to operate at the
Conn level.  Teardown is now guaranteed to happen regardless of abort or close.

There's no change in exported API, save for a new ConnOption: ConnLog.  ConnLog
sets the connection's logging destination.

The PingPong benchmark shows some small performance gains, but nothing
substantial.  Future work will be to reduce the number of allocations made.