per-remote metadata storage

Actually very straightforward reuse of the metadata log file code.
Although I had to add a todo item as git-annex forget won't clean up
dead remote's metadata yet.

This would be worth adding to the external special remote interface
sometime. Have not opened a todo though, guess I'll wait until something
needs it.

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

Command/MetaData.hs

@@ -112,7 +112,7 @@ perform c o k = case getSet o of
 	Set ms -> do
 		oldm <- getCurrentMetaData k
 		let m = combineMetaData $ map (modMeta oldm) ms
-		addMetaData' k m c
+		addMetaDataClocked k m c
 		next $ cleanup k
 	_ -> next $ cleanup k