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

- fix a flacky test 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
Consistency check on entries when opening a CountsStore
Create both alpha and beta counts store files on creation
On startup pick the non corrupted counts store file with the highest version
Check that the chosen counts store is not too old wrt the other stores/log

- fix: when writing the new counts store make sure we pad with zeros up to the length of the file
Code cleanup and documentation
Record size should be a parameter to the generic KV-store
KeyValueRecordVisitor should accept a register from outside rather than allocate one itself