Merge changes I25fb5fce,I86cffff6,I031eb1a1,Ie40633c0,I08562b89

* changes:
  snapuserd: Sort REPLACE ops for batch merge
  snapuserd: API to query snapshot and merge status
  snapuserd: Wire up API's for Initiating and tracking Merge
  snapuserd: I/O requests which are not block aligned.
  snapuserd: Service I/O requests from dm-user