tabletserver: add support for tx pool timeout

We want to fail early, but not immediately, if the
transaction pool is full. So, we're adding a separate
timeout for transaction pool waits.
queryservice_test: add test for tx pool available
Merge pull request #89 from youtube/suguwork

tabletserver: add support for tx pool timeout
Changed the test to do an explicit rollback incase of an exception.
Fixed integrity_error_logging test to rollback the transaction.