Use 'default' keyword in Coprocessor Observer interfaces to get rid of 'Base...Observer' implementations.

In cases where one might need to use multiple observers, say region, master and regionserver; and the fact that only one class can be extended, it gives rise to following pattern:

    public class BaseMasterAndRegionObserver
      extends BaseRegionObserver
      implements MasterObserver

    class AccessController
      extends BaseMasterAndRegionObserver
      implements RegionServerObserver

were BaseMasterAndRegionObserver is full copy of BaseMasterObserver.

There is an example of simple case too where the current design fails.
Say only one observer is needed by the coprocessor, but the design doesn't permit extending even that single observer (see RSGroupAdminEndpoint), that leads to full copy of Base...Observer class into coprocessor class leading to 1000s of lines of code and this ugly mix of 5 main functions with 100 useless functions.