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.
No, if a lock is not session scoped, it is open-scoped. See section 17.6 of the [JCR 2.0 specification](http://www.day.com/maven/jcr/2.0/17_Locking.html#17.6%20Session-Scoped%20and%20Open-Scoped%20Locks):

> When a lock is placed on a node, it can be specified to be either a session-scoped lock or an open-scoped lock. A session-scoped lock automatically expires when the session through which the lock owner placed the lock expires. An open- scoped lock does not expire until it is explicitly unlocked, it times out or an implementation-specific limitation intervenes.