Lease recovery, concurrency support.

Lease recovery happens if hdfs client does not renew its leases within an hour. Lease recovery triggers block recovery if the block has not been completed during normal write or append operations. Name-node initiates such block recovery by sending BlockRecoveryCommand to data-nodes (via reply to a heartbeat). There could be a race condition if some data-nodes still recover the block when the name-node decides to start a new recovery because the previous one did not complete. This issue should take of the race condition by introducing recovery ids. It should also implement the new recovery protocol, which is described in the append design document,