Throttle for streaming queries.

Also includes changes to semaphore to support timeouts.
Throttle for streaming queries.

Also includes changes to semaphore to support timeouts.
Fix vtbarnacle, add delay support in sandbox.
Merge branch 'stream-throttle'
Merge branch 'master' into barnacle
Barnacle unit tests done.
Better error messages.
Comments for all tests.
Report deadlocks as separate error stats.
Move error constants into mysql package.
Rename barnacle -> vtgate
Fixing BSON encoding of SrvKeyspace.