Make vttablet.InitAgent return the agent.
Merge branch 'export-agent'
Fix scope error.
Undo some ineffective optimizations.

Allow streaming queries to complete on shutdown.
Don't throttle streaming queries till first row return. It doesn't
help.
Merge branch 'better-timeouts'
Stats & logging for UpdateStream

UpdateStream now reports:
- Number of streams it's running
- Number of errors it's encountered
- Number of statements or events parsed
- Number of transactions

Also took care of some misc TODOs.
vttablet error stats recategorization.

InfoErrors: Expected failures from normal app flows, like dup key.
Errors: Errors that cause actual failures in the app. A high rate here
should generate a page.
InternalErrors: Unexpected internal errors, that should generate an
alert (not a page), which need to be escalated to the corresponding
developers.
Merge branch 'master' into replication