Begin storing zk-monitor cluster state in Zookeeper.

Through a common cluster.State() object, zk-monitor can now register
itself as a monitoring agent in a farm of zk-monitors. This object
provides a single common access point for writing cluster state to
Zookeeper.

The Alerter 'lock' code has been simplified and now a common getLock()
method in the State() object provides a way to get properly sandboxed
locks quickly and simply.