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).
No `logout` here, either?