libsnapshot:snapuserd: Snapshot merge support.

Handle write IO during snapshot merge. When merge
is completed, dm-snapshot-merge issues WRITE IO
to indicate the completion of merge for a specific
operation. Snapuserd daemon tracks the merge completion
process in the COW header; this is required to handle
resuming merge operation gracefully if there is a crash
during merge.

Bug: 168311203
Test: vts_libsnapshot_test, cow_snapuserd_test
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I4fc8cc23a6ec5b26b7ae3339212efdd3b9a367e8