Fix transfer log file locking problem when running concurrent transfers.

orElse is great, but was not the right thing to use here because
waitTakeLock could retry for other reasons than the lock being held,
which made tryTakeLock fail when it shouldn't.

Instead, move the code to tryTakeLock and implement waitTakeLock using
tryTakeLock and retry.

(Also, in runTransfer, when checkSaneLock fails, dropLock to avoid leaking a
lock handle.)

This commit was supported by the NSF-funded DataLad project.

Annex/Transfer.hs

@@ -118,7 +118,9 @@ runTransfer' ignorelock t afile shouldretry transferaction = checkSecureHashes t
 					void $ liftIO $ tryIO $
 						writeTransferInfoFile info tfile
 					return (Just lockhandle, False)
-				, return (Nothing, True)
+				, do
+					liftIO $ dropLock lockhandle
+					return (Nothing, True)
 				)
 #else
 	prep tfile _mode info = catchPermissionDenied (const prepfailed) $ do