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.
@rhauch: i think this is the correct place because the `lockTokens` are part of the `JcrLockManager` class (not the `RepositoryLockManager` class).
However, I think the code should use an iterator of the keySet of the `ConcurrectHashMap` as opposed to copying it into a List. I see no benefit of the copying.