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.
@hchiorean, great suggestion re iterating over the existing concurrent map rather than copying the contents. @mikedurbin, can you make that change?