use latest meshy snapshot
query file ref cache bug fixes and refactoring

- removed FileReferenceWrapper, which appears to have been missing
  size and modification time in its equals method. That would have
  caused collisions in the Set<FileReferenceWrapper> objects and
  prevented the "use only the most recent file refs" logic from
  functioning. Also, they seem to have been pretty redundant, so it
  is cheaper and simpler to do away with them.

- prevent many non-thread-safe modifications of nested sets of
  FileReference(Wrapper)s. It is not okay to modify them just
  because the enclosing Map is thread safe.

- prevent file ref cache from ever returning null

- fix up file ref gathering time-out logic to support short
  circuiting in a sane manner

- use Multimap over Map<?, Collection<?>>

- use daemon threaded executor services over "exiting" services
  to prevent needless shutdown hooks

- break FileRefCache nested classes up into static non-nested
  classes

- use CountDownLatch + StampLock over not-actually-threadsafe
  ConcurrentHashMap<?, Collection<?>> and unnecessary semaphore