Fix: fencing: Record the last known names of nodes to make sure fencing requested with nodeid works

dlm_controld invokes dlm_stonith which calls stonith_api_kick_helper()
to request fencing by nodeid. We need to translate the nodeid into
the nodename when handling the fencing. Previously if the node had been
lost, fencing would fail since the peer cache entry had been autoreaped.

This commit fixes it by recording the last known names of the nodes in a
dedicated hash table. This commit also switches the invocations of
crm_get_peer() to crm_find_peer() to avoid recreating a node entry in the
peer cache.