Clustering : Make persistence configurable at the datastore or shard level

Operational data does not need to be persisted. We should therefore be able to disable persistence for the whole operational datastore or a maybe even specific shards so that we can use this as a means to optimize performance.

Currently we persist by default to ensure that when a node is restarted it can restore it's state from disk and subsequently get synced up with the Leader. If we do not restore the state from disk the complete state will be replicated from the leader. We need to ensure that replicating the whole state of a shard is fast enough so that using replication vs persistence for recovery does not have an adverse effect on performance.