Fix OOIION-724. Harden Event Persister

This PR makes the event_persister process retry persisting any events that previously failed persisting, before taking new events off the queue. This is needed in testing, because the "events" datastore gets dropped and recreated for each new test.

Also, the event_persist_interval is now configurable when the event_persister is spawned.

And some better log statements and comment improvements.