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.)

P2P/Proxy.hs

@@ -54,7 +54,7 @@ closeRemoteSide remoteside =
  - actions.
  - -}
 data ProxySelector = ProxySelector
-	{ proxyCHECKPRESENT :: Key -> Annex RemoteSide
+	{ proxyCHECKPRESENT :: Key -> Annex (Maybe RemoteSide)
 	, proxyLOCKCONTENT :: Key -> Annex (Maybe RemoteSide)
 	, proxyUNLOCKCONTENT :: Annex (Maybe RemoteSide)
 	, proxyREMOVE :: Key -> Annex RemoteSide
@@ -64,7 +64,7 @@ data ProxySelector = ProxySelector
 
 singleProxySelector :: RemoteSide -> ProxySelector
 singleProxySelector r = ProxySelector
-	{ proxyCHECKPRESENT = const (pure r)
+	{ proxyCHECKPRESENT = const (pure (Just r))
 	, proxyLOCKCONTENT = const (pure (Just r))
 	, proxyUNLOCKCONTENT = pure (Just r)
 	, proxyREMOVE = const (pure r)
@@ -160,9 +160,13 @@ proxy proxydone proxymethods servermode (ClientSide clientrunst clientconn) prox
 
 	proxyclientmessage Nothing = proxydone
 	proxyclientmessage (Just message) = case message of
-		CHECKPRESENT k -> do
-			remoteside <- proxyCHECKPRESENT proxyselector k
-			proxyresponse remoteside message (const proxynextclientmessage)
+		CHECKPRESENT k -> proxyCHECKPRESENT proxyselector k >>= \case
+			Just remoteside -> 
+				proxyresponse remoteside message
+					(const proxynextclientmessage)
+			Nothing -> 
+				protoerrhandler proxynextclientmessage $
+					client $ net $ sendMessage FAILURE
 		LOCKCONTENT k -> proxyLOCKCONTENT proxyselector k >>= \case
 			Just remoteside -> 
 				proxyresponse remoteside message