Fix for inability to remove constraints in HA.

 o Added test that verifies database does not block writes while populating constraint index.
 o Verifies that constraint deletion works in HA clusters.
 o Added test to verify new slave joining cluster properly initializes constraints
 o Added before/after records for schema commands to allow deleting schema rules from
   logical log.
Constraints are now stored and read from the schema cache.
Merge pull request #1231 from jakewins/ha-constraints

Fix for inability to remove constraints in HA.