Implementing snapshot schedule to avoid high latency issue due to disk contention

If ZK server is running a machine with single disk driver, the snapshot and txn fsync thread will have disk IO contention (even on SSD). Majority taking snapshot will affect the txn fsync time, and hence the end to end update and read latency.

To provide better SLA guarantee and improve the write throughput with large snapshot (> 3GB), the snapshot scheduler is implemented internally to avoid majority taking snapshot at the same time, which provides better latency guarantee.

A new quorum packet type SNAPPING is introduced in this feature, leader will send this packet to the followers periodically like PING but less frequently. Followers will send the current status back, like the maximum txns since last snapshot, fsync latency, etc, and leader will decide who should take snapshot.

On follower, it will enable safe snapshot mode if leader is sending SNAPPING, which will only take snapshot if the txns is much larger than the threshold we defined for SyncRequestProcessor, this is used to avoid issues like the follower accumulated too many txns before it is scheduled to take snapshot.