Indexer bug fixes

1. Revert to single insert: Bulk insert will fail the entire batch if a
   duplicate key exception is encountered for any document. This is causing
   almost half the postal code features to not be imported.
2. Synchronization fix: Countdown revgeo latch only after all revgeo coverage
   workers are done rather than as soon as Done message is received from
   PolygonLoader

Auditors: nsanch, blackmad