Cleanup and improvements in IndexStatisticsTest

- improve assertions in IndexStatisticsTest to be more precise
- fix tests failures in CI
- add UpdatesTracker to maintain counts of updates happened during or after population
- add a test for heavy concurrent updates during population
Log index sampling
Use a fair tryLock call in IndexSamplingController

- improve handling of InterruptedException in DoubleLatch
Log when rebuilding counts on startup
Log when flushing the schema state
Tentative fix of UniqueConstraintStressIT flakyness