Fixes for some flaky tests

o Safeguard against NullPointerException in ConstraintVerificationFKE, by
  using evidence() method to get it.
o Fix LuceneIndexRecoveryIT.shouldNotAddTwiceDuringRecoveryIfCrashedDuringPopulation
   - Test was not deterministic about completion of population job
   - Cleaned up/reverted changes in FlippableIndexProxy made because of the above issue