db/update: convert to OO API

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

src/Instance.hxx

@@ -29,6 +29,7 @@
 class NeighborGlue;
 #endif
 
+class UpdateService;
 class ClientList;
 struct Partition;
 
@@ -42,6 +43,8 @@ struct Instance final
 	NeighborGlue *neighbors;
 #endif
 
+	UpdateService *update;
+
 	ClientList *client_list;
 
 	Partition *partition;