Revert "Make index population job not flip during shutdown"

This reverts commit 7edfe46550259adc1f28792ddcade3322e137f0f.
Safeguard against NullPointerException in ConstraintVerificationFKE

o Use evidence() instead of evidence when looping in getUserMessage()
Fix LuceneIndexRecoveryIT.shouldNotAddTwiceDuringRecoveryIfCrashedDuringPopulation

o Test was not deterministic about completion of population job
o Cleaned up/reverted changes in FlippableIndexProxy made because of the above issue
o Made ControlledPopulationSchemaIndexProvider optionally take a delegate