Add --max-in-flight argument to vtgate.

This is a means to limit the amount of queueing which can occur, which
should cause failure states to report as such faster rather than
becoming sinks for queries.
Fixup unit test.
Add new client-side error type to represent request backlog errors.

fixup
Merge pull request #78 from plobsing/vtgate-max-inflight

Vtgate max inflight