Snapshot Create requires Double Buffer Flush thread to split the commit batch

The OmRequest double buffer flush thread flushes the entire buffer as a batch.  Since follower OM's will flush batches with different contents, snapshots can't stay consistent between the leader and the followers.

This means the flush thread needs to be "snapshot aware" and split the batch so that all operations before the snapshot create are commited to rocksdb before the checkpoint is created.

Details here:
https://docs.google.com/document/d/18BRPMol3EX5FioRaHliksx5uIGTw5iYTFc83PAKFLQU/edit#