Make replication queue initialization asynchronous

Today, initialization of replication queues blocks safe mode exit and certain HA state transitions. For a big name space, this can take hundreds of seconds with the FSNamesystem write lock held. During this time, important requests (e.g. initial block reports, heartbeat, etc) are blocked.

The effect of delaying the initialization would be not starting replication right away, but I think the benefit outweighs. If we make it asynchronous, the work per iteration should be limited, so that the lock duration is capped.

If full/incremental block reports and any other requests that modifies block state properly performs replication checks while the blocks are scanned and the queues populated in background, every block will be processed. (Some may be done twice) The replication monitor should run even before all blocks are processed.

This will allow namenode to exit safe mode and start serving immediately even with a big name space. It will also reduce the HA failover latency.