librbd: track in-progress migration aborting operation

librbd: track in-progress migration aborting operation

We want to prevent the destination image from being used while an
abort is in-progress. Test that the image has no watchers prior to
permitting the abort, switch the migration state to ABORTING, and
treat the image as read-only if the migration state is ABORTING.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit d848b4f1c083bd9f3e6eac3e186c9e7df2e22004)
(cherry picked from commit 2bc5229717326298ca964333fc07a38c5e48701e)

Conflicts:
	src/librbd/image/RefreshRequest.cc: lock and read-only flags refactor