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).
as explained in the description of the PR, internal session should never be logged out because that will invalidate the context of the original session. `restoreSession` is such an example. 
For example, if the context of the original session contained an authenticated user, logging out the `restoreSession` would log out/invalidate the security context.