fix transfers of key with no associated file

Several places assumed this would not happen, and when the AssociatedFile
was Nothing, did nothing.

As part of this, preferred content checks pass the Key around.

Note that checkMatcher is sometimes now called with Just Key and Just File.
It currently constructs a FileMatcher, ignoring the Key. However, if it
constructed a FileKeyMatcher, which contained both, then it might be
possible to speed up parts of Limit, which currently call the somewhat
expensive lookupFileKey to get the Key.

I have not made this optimisation yet, because I am not sure if the key is
always the same. Will need some significant checking to satisfy myself
that's the case..

Assistant/Threads/TransferScanner.hs

@@ -164,9 +164,9 @@ expensiveScan urlrenderer rs = unless onlyweb $ batch <~> do
 			let slocs = S.fromList locs
 			let use a = return $ mapMaybe (a key slocs) syncrs
 			ts <- if present
-				then filterM (wantSend True (Just f) . Remote.uuid . fst)
+				then filterM (wantSend True (Just key) (Just f) . Remote.uuid . fst)
 					=<< use (genTransfer Upload False)
-				else ifM (wantGet True $ Just f)
+				else ifM (wantGet True (Just key) (Just f))
 					( use (genTransfer Download True) , return [] )
 			let unwanted' = S.difference unwanted slocs
 			return (unwanted', ts)