util.adminstream: Set a read timeout handler

So that net.server doesn't close the connection on inactivity.

util/adminstream.lua

@@ -247,6 +247,11 @@ local function new_server(sessions, stanza_handler)
 			sessions[conn]  = nil;
 		end
 	end
+
+	function listeners.onreadtimeout(conn)
+		conn:send(" ");
+	end
+
 	return {
 		listeners = listeners;
 	};
@@ -315,6 +320,10 @@ local function new_client()
 		client.conn = nil;
 	end
 
+	function listeners.onreadtimeout(conn)
+		conn:send(" ");
+	end
+
 	client.listeners = listeners;
 
 	return client;