ctdb-recovery: Rename recovery lock functions and struct

ctdb-recovery: Rename recovery lock functions and struct

Use the more general name "cluster mutex", since we are likely to end
up with more than one cluster-wide lock.  There will probably be a
dedicated recovery lock, held only during recovery, and also a second
lock that is held by the master node.  Currently one lock is used for
both purposes.

At the moment the struct and functions are involved with setting the
recovery mode.  However, they'll be abstracted out to more generally
deal with the cluster mutexes, so "recmode" -> "cluster_mutex".  Drop
"set" from names, since this is used to test the lock.  Also drop
"ctdb" prefix from functions, since they are local to this file.  The
struct will eventually be a long-lived handle that will release the
mutex when freed, so name it accordingly.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>