Fix or add Index and BlobStore tests with SystemTime rather than MockTime of 0

Most of the tests in Index and BlobStore are using a MockTime and uses 0 as it initial value. So, chances are that integer overflow may not be caught with such tests when SystemTime is used. Would be good to initialize the MockTime to SystemTime.currentTimeInmillis() so that we can mimic the real system.

Locate the tests for Index and BlobStore and inspect how MockTime is initialized. Compare those setups with SystemTime.currentTimeInmillis(), then update or add coverage so time-dependent integer overflow behavior is exercised with a realistic starting time.

Written by the indexing model from the issue text.