DataNode restarts may introduce corrupt/duplicated/lost replicas when handling detached replicas

Current trunk first calls detach to unlinks a finalized replica before appending to this block. Unlink is done by temporally copying the block file in the "current" subtree to a directory called "detach" under the volume's daa directory and then copies it back when unlink succeeds. On datanode restarts, datanodes recover faied unlink by copying replicas under "detach" to "current".

There are two bugs with this implementation:
1. The "detach" directory does not include in a snapshot. so rollback will cause the "detaching" replicas to be lost.
2. After a replica is copied to the "detach" directory, the information of its original location is lost. The current implementation erroneously assumes that the replica to be unlinked is under "current". This will make two instances of replicas with the same block id to coexist in a datanode. Also if a replica under "detach" is corrupt, the corrupt replica is moved to "current" without being detected, polluting datanode data.