re-subscribe logic

1. Re-subscribe logic in registry handler that loops subscription logic for 3 times
2. Addresses gson serialization issue by providing a simple typeadapter for handling fields of type 'class' as seen in 'Modify' -type change notification.
Every subscription (triggered by retry in this case) re-runs the source. So when you retry, eureka client should give you the latest stream, which may be from a different read server or not, this code should not be concerned about that, it should only be concerned about the stream of the specified interest.