[CURATOR-470] Add new service cache listener to let users know what instances have changed

[Randgalt]

Built off of #265

A new interface (ServiceCacheEventListener) is introduced to expose changed instances, while preserving backward compatibility.

Unit test has updated as well.

@JerryChin this PR builds on yours but is more backward compatible. I'm concerned that the new listener breaks the contract of the old one. So, the cache now maintains two separate listener containers.