Add a close() method to ZooKeeperMap.

Enable a ZooKeeperMap user to signal that it no longer requires the map
any longer by calling ZooKeeperMap.close(). This removes any persistent
watchers against ZooKeeperClient (allowing it to be GC'd), stops
registering any additional watchers against ZooKeeper, and prevents any
additional callbacks to the ZooKeeperMap.Listener interface.