misc: rename safelease to clusterlock

The safelease module is now contaning also the sanlock implementation
and soon it might contain other (e.g.: a special lock for local storage
domains), for this reason it has been renamed with a more general name
clusterlock. The safelease implementation also required some cleanup in
order to achieve more uniformity between the locking mechanisms.

Change-Id: I74070ebb43dd726362900a0746c08b2ee3d6eac7
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10067
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>