Refactored the YarnSchedulerInterceptor class hierarchy.
 - Shrunk the interface itself. Got rid of the EventListener inner interface.
 - Implemented a CompositeInterceptor.
    - It wraps a bunch of interceptors.
    - It initializes myriad via MyriadInitializationInterceptor.
    - It allows more interceptors to be registered into it via a Registry interface.
- Implemented a BaseInterceptor which allows child interceptors to selectively override methods that they care about.

Tested and verified that myriad's flex up and flex down ops work as expected.