Add RealmAwareZkClient and RealmAwareZkClientFactory #760
Description
RealmAwareZkClient and RealmAwareZkClientFactory are going to be the top-level interfaces for other realm-aware ZkClient APIs (think FederatedZkClient, DedicatedZkClient, etc.).
RealmAwareZkClient will support all the existing interface methods that HelixZkClient supports.
RealmAwareZkClientFactory will be the interface implemented by SharedZkClientFactory and DedicatedZkClientFactory.