When running `make test`:
The "java" and "local_example" tests failed due to not having `mvn`, so I needed to install the "maven" package.
The "vtctld_web" test failed due to "Can't start Xvfb ... No such file or directory" and "WebDriverException: Message: unknown error: cannot find Chrome binary, so I needed to install the "xvfb" and "chromium-browser" packages.

changes requested by @michael-berlin

pull

merge

The purpose of this limiter is to prevent single user from consuming
whole transaction pool.

BUG=69448559

Work in progress: This is only a proposal for how to integrate TxLimiter
with TxEngine/TxPool. It's obviously not ready yet, just need this
reviewed whether it's valid approach.

The timing is both unreliable for tests and not useful in general, so
just make the ExecTime 0 for all plans.

Both the addition of stats for the plans and the recent vindex changes
for bind variables changed the vtexplain output, so update the expected
output accordingly.

Instead of outputting the full json / text output emitted by the test,
only output the diffs.

Also include a hint to the developer to inform how to update the expected
test output files from the temp output of the given run.

… delete; no subpartition support

support explicit partition clause

…gain

Fix vtexplain tests again

Disable query cache in default.cnf

add an option to enable grpc tracing

I noticed when running
```
go test 'github.com/youtube/vitess/go/vt/vtgate' -v
```

these errors from glog:

> ERROR: logging before flag.Parse: I0104 16:13:49.401758   22951 buffer.go:144] vtgate buffer not enabled.
> ERROR: logging before flag.Parse: I0104 16:13:49.401895   22951 discoverygateway.go:103] loading tablets for cells: 
> ERROR: logging before flag.Parse: I0104 16:13:49.401907   22951 gateway.go:84] Gateway waiting for serving tablets...
> ERROR: logging before flag.Parse: I0104 16:13:49.401925   22951 gateway.go:91] Waiting for tablets completed
> ERROR: logging before flag.Parse: I0104 16:13:49.401949   22951 vtgate.go:68] Transaction mode: 'MULTI'
> ERROR: logging before flag.Parse: I0104 16:13:49.402435   22951 streamlog.go:141] Streaming logs from VTGate at /debug/querylog.

This trick from kubernetes/kubernetes#17162 can suppress them, if you want.

no comment

suppress errors for package vtgate tests

Add "init_config" option to mysqlctl.

export the grpc version as a debug var

add a flag to enable query logging to a file for vtgate

vindex show statements

drain mysql protocol connections to vtgate at shutdown

This makes API smaller and cleaner.

* Remove unused 'method string' from Executor.execute().
* Remove srv_topo_timeout flag from resilient_srv_topo_server.
  We can just used the provided context, no need to slap on
  a random timeout.
* Update MessageStream comment, we do support multiple shards.
* In vtgate SplitQuery, use closures directly, inside of wrapping them
  in a method.

The new package will also eventually contain:
* the ResilientSrvTopoServer implementation of that interface.
* helper methods to resolve cell/keyspace/shards.

It's more consistent to have the watch code in srvtopo.

To clarify what is going on in the two cases of inside-a-transaction
and not-in-a-transaction, explicitly enumerate all the expected plan
opcodes and make the default case an error, rather than relying on
default behavior.

…arning

remove the verbose log when fieldinfo can't be cached

modify some comments

Transaction pool limits

* SrvVSchema update was not thread-safe. Now it is protected by the
  mutex.
* Added a unit test for srvtopo.Server.WatchSrvVSchema.

…ranularity

increase the frequency of vtgate qps rate metrics from 1m to 5s

A 'not found' is not a vschema watch error any more.

Vtgate refactors.

query executor cleanup