ParticipationRegistry In-Memory Cache

[winder]

The ParticipationRegistry should have an in-memory cache to optimize read and writes.

1. create a cache of ParticipationRecord objects.
2. on start load all records into the cache.
3. update the record function to modify the cache and skip the write.
4. new Flush command in the interface called in "oldKeyDeletionThread" to update the DB each round.