Rational:
* When the config is reloaded and the provider context is canceled, we need to
exit the current ZK `TargetProvider.Run` method as a new provider will be
instantiated.
* In case `Stop` is called on the `ZookeeperTreeCache`, the update/events
channel may not be closed as it is shared by multiple caches and would
thus be double closed.
* Stopping all `zookeeperTreeCacheNode`s on teardown ensures all associated
watcher go-routines will be closed eagerly rather than implicityly on
connection close events.
