Align GCMonitorMBean MBean with new generation based GC

The GCMonitorMBean MBean still dates back to the old oak-segment. We need to review its endpoints and only keep those that make sense for oak-segment-tar, adapt the others as necessary any add further functionality as required.

Specifically I think we should get rid of the time series for getRepositorySize() and getReclaimedSize().

Also the name getRepositorySize() is confusing and we should change it. It leads callers to think it would return current size of the repository opposed to the size it had after the last cleanup. (There is FileStoreStatsMBean.getRepositorySize() for the latter.)