snapshot: rework cluster creation and removal

snapshot: rework cluster creation and removal

Support for exclusive activation of snapshots revealed some problems.

When snapshot is created, COW LV is activated first (for clearing) and
then it's transformed into snapshot's COW LV, but it has left the lock
for such LV active in cluster and this lock could not have been removed
from dlm, unless snapshot has been removed within same dlm session.

If the user tried to remove snapshot after rebooting node, the lock was
missing, and COW LV could not have been detached.

Patch modifes the approach in this way:

Always deactivate COW LV for clustered vg  after clearing (so it's
activated again via imlicit snapshot activation rule when snapshot is activated).

When snapshot is removed, activate COW LV as independend LV, so the lock
will exist for such LV, but only when the snapshot is active.

Also add test case for testing snapshot removal after cluster reboot.