librbd: deep-copy snapshots from a specified start/end position

librbd: deep-copy snapshots from a specified start/end position

Allow the snapshots to be arbitrarily copied from any source image
start/end snapshot ids. If the end snapshot is not a user-snapshot,
it will associate to the destination image HEAD revision.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 1fac1f72c2d2000175f854f9bb14ec95ccb68b08)

Conflicts:
	src/librbd/deep_copy/SnapshotCopyRequest.cc: different lock types
	src/test/librbd/deep_copy/test_mock_SnapshotCopyRequest.cc: no mirror snapshot namespaces