Amend HBASE-11497 Expose RpcScheduling implementations as LimitedPrivate interfaces (Matteo Bertozzi)

Changes RpcScheduler from an interface into an abstract class
HBASE-11513 Combine SingleMultiple Queue RpcExecutor into a single class (Jesse Yates)
HBASE-11367 Pluggable replication endpoint
HBASE-11442 ReplicationSourceManager doesn't cleanup the queues for recovered sources (Virag Kothari)
HBASE-4495 CatalogTracker has an identity crisis; needs to be cut-back in scope (ADDENDUM patch to fix flaky unit test)