Use a lock to serialize access to a shared reference to ZooKeeperWatcher in HBaseReplicationEndpoint

The code that closes down a ZKW that fails to initialize when attempting to connect to the remote cluster is not MT safe and can in theory leak ZooKeeperWatcher instances. The allocation of a new ZKW and store to the reference is not atomic. Might have concurrent allocations with only one winning store, leading to leaked ZKW instances.