Thread leaked by fdb_close

In some circumstances ForestDB can leak pthreads; the thread sanitizer in the new Xcode 8 beta reports this in a few of my unit tests.

It looks like the problem occurs when:

• filemgr_prefetch creates a new thread and stores its handle in file->prefetch_tid (filemgr.cc:628)
• filemgr_free_func is called when the database closes, but its prefetch_state is not FILEMGR_PREFETCH_RUNNING, so it does not call thread_join on the prefetch_tid. (filemgr.cc:1370-1375)

There are no other API calls that use prefetch_tid, so nothing else will terminate that thread.

NOTE: I'm on forestdb commit fd41b293, from the stable branch as of 4/23.