close pid lock only once no threads use it

This fixes a FD leak when annex.pidlock is set and -J is used. Also, it
fixes bugs where the pid lock file got deleted because one thread was
done with it, while another thread was still holding it open.

The LockPool now has two distinct types of resources,
one is per-LockHandle and is used for file Handles, which get closed
when the associated LockHandle is closed. The other one is per lock
file, and gets closed when no more LockHandles use that lock file,
including other shared locks of the same file.

That latter kind is used for the pid lock file, so it's opened by the
first thread to use a lock, and closed when the last thread closes a lock.

In practice, this means that eg git-annex get of several files opens and
closes the pidlock file a few times per file. While with -J5 it will open
the pidlock file, process a number of files, until all the threads happen to
finish together, at which point the pidlock file gets closed, and then
that repeats. So in either case, another process still gets a chance to
take the pidlock.

registerPostRelease has a rather intricate dance, there are fine-grained
STM locks, a STM lock of the pidfile itself, and the actual pidlock file
on disk that are all resolved in stages by it.

Sponsored-by: Dartmouth College's Datalad project

Utility/LockFile/PidLock.hs

@@ -274,7 +274,7 @@ waitLock (Seconds timeout) lockfile displaymessage sem = go timeout
 			liftIO $ sem False
 			waitedLock (Seconds timeout) lockfile displaymessage
 
-waitedLock :: MonadIO m => Seconds -> PidLockFile -> (String -> m ()) -> m LockHandle
+waitedLock :: MonadIO m => Seconds -> PidLockFile -> (String -> m ()) -> m a
 waitedLock (Seconds timeout) lockfile displaymessage = do
 	displaymessage $ show timeout ++ " second timeout exceeded while waiting for pid lock file " ++ fromRawFilePath lockfile
 	giveup $ "Gave up waiting for pid lock file " ++ fromRawFilePath lockfile