Cluster-aware lock expiration

The current lock expiration mechanism is only able to expire locks on the cluster node where they were first added. This avoids nasty race conditions, but hides the expiration time (getRemainingTime) from other cluster nodes and breaks expiration of the lock if the originating cluster node is removed from the cluster.

It would be nice to have a good cluster-wide lock expiration mechanism that doesn't end up with all cluster nodes trying to unlock expired locks at the same time.