xmpp: send a presence query when there's an important message to send

This may work around google talk's horrible presence handling, in which
clients often don't learn about other clients, at least when using the same
account. This way, every time we start a git push over xmpp, we'll waste
bandwidth asking clients to please try again to identify themselves.

Assistant/Threads/XMPPClient.hs

@@ -184,7 +184,8 @@ relayNetMessage selfjid = do
   where
 	handleImportant msg = case parseJID =<< isImportantNetMessage msg of
 		Just tojid
-			| tojid == baseJID tojid ->
+			| tojid == baseJID tojid -> do
+				putStanza presenceQuery
 				storeImportantNetMessage msg (formatJID tojid) $
 					\c -> (baseJID <$> parseJID c) == Just tojid
 		_ -> noop