Make updates to the Finder caches atomic. Well, almost.

Make updates to the Finder caches atomic.  Well, almost.

Flushing and uncaching a single module is not completely atomic since
both caches a cleared separately.  However, flushing is only done when
changing the working directory which shouldn't be done concurrently to
other threads.  Uncaching is only done in 'summariseModule' which
requires some more work to become thread-safe anyway.