tablet.py refactoring phase 1

Added a start_vtocc function. It works, but there's still
a lot of duplicate code.
queryservice_test revamp test_env getting better
queryservice_test: merged TestEnv classes into one
queryservice_test: final touches
Merge branch 'rowcache'
Adding a mutex lock in an extra place to make thread sanitizer happy.
Merge branch 'master' into replication