Merge changes I5e93fcbf,I9d4c8bcf,Icd580aae,I7b2399a4

* changes:
  libsnapshot: Implement merge flow.
  libsnapshot: Unmap COW devices when deleting snapshots.
  libsnapshot: Eliminate per-snapshot flocks.
  libsnapshot: Improve first test-run and test cleanup.