MODE-2342: Remove lock tokens from sessions upon logout().

When a session ends (via logout()) any lock tokens held by that session were left in a state where they couldn't be associated with another session (or removed from the terminated session).

This small change (and illustrative test) changes that behavior in a way that is not inconsistent with the JCR spec and simplifies use of locks in sessions.
This looks good. @hchiorean, do you think this is better off inside the `lockManager.cleanLocks(session)` call, which is where we do the other cleanup? IMO, it could go either place.