MODE-2343 Removed the EVENT related statistics from the JcrObservationManager and fixed the cleanup code required when a session is terminated (either via logout or repository shutdown)

Fixing the cleanup required by `session.logout` exposed another "interesting" issue: internal sessions (created via `session.spawnSession`) should never be logged out because doing do would invalidate their context (and therefore the context of the owner session).
These two lines should probably be:

    if ( this.repositoryObservable.register(adapter) ) this.listeners.add(adapter);

just in case we can't register the adapter (for whatever reason).