checkpresent support for clusters

This assumes that the proxy for a cluster has up-to-date location
logs. If it didn't, it might proxy the checkpresent to a node that no
longer has the content, while some other node still does, and so
it would incorrectly appear that the cluster no longer contains the
content.

Since cluster UUIDs are not stored to location logs,
git-annex fsck --fast when claiming to fix a location log when
that occurred would not cause any problems. And presumably the location
tracking would later get sorted out.

At least usually, changes to the content of nodes goes via the proxy,
and it will update its location logs, so they will be accurate. However,
if there were multiple proxies to the same cluster, or nodes were
accessed directly (or via proxy to the node and not the cluster),
the proxy's location log could certainly be wrong.

(The location log access for GET has the same issues.)

Annex/Cluster.hs

@@ -60,13 +60,8 @@ clusterProxySelector clusteruuid protocolversion = do
 		<$> remoteList
 	remotesides <- mapM (proxySshRemoteSide protocolversion) remotes
 	return $ ProxySelector
-		{ proxyCHECKPRESENT = \k -> error "TODO"
-		, proxyGET = \k -> do
-			locs <- S.fromList <$> loggedLocations k
-			case filter (flip S.member locs . remoteUUID) remotesides of
-				-- TODO: Avoid always using same remote
-				(r:_) -> return (Just r)
-				[] -> return Nothing
+		{ proxyCHECKPRESENT = nodecontaining remotesides
+		, proxyGET = nodecontaining remotesides
 		, proxyPUT = \k -> error "TODO"
 		, proxyREMOVE = \k -> error "TODO"
 		-- Content is not locked on the cluster as a whole,
@@ -75,3 +70,11 @@ clusterProxySelector clusteruuid protocolversion = do
 		, proxyLOCKCONTENT = const (pure Nothing)
 		, proxyUNLOCKCONTENT = pure Nothing
 		}
+  where
+	nodecontaining remotesides k = do
+		locs <- S.fromList <$> loggedLocations k
+		case filter (flip S.member locs . remoteUUID) remotesides of
+			-- TODO: Avoid always using same remote
+			(r:_) -> return (Just r)
+			[] -> return Nothing
+