Voltron: Last reference removed.

Voltron was Vitess's original project name, and this was
the last reference of that name. Long live!
ActiveTxPool -> TxPool
rowcache: concurrentize delayed spot-checking

If a spot check fails, the rowcache sleeps for 10 seconds
and rechecks the data. This is to accommodate race conditions
that inherently exist between rowcache and database.
Such sleeps increase query execution time. With this CL,
the recheck is launched as a separate goroutine using the
new Launch service of QueryEngine which will ensure that
it won't be closed until such functions have completed.
tabletserver: add panic failsafe for tasks
tabletserver: move taks.Wait to Close.

There's a subtle race condition. Queries that are currently
executing can launch new tasks after WaitForTxEmpty has
returned. So, the tasks.Wait should only be done after
all existing queries have completed, by which time no new
tasks will be launched.
Merge branch 'suguwork'