MINOR: server: generalize the "updater" source

the function server_parse_addr_change_request() contain an hardcoded
updater source "stats command". this function can be called from other
sources than the "stats command", so this patch make this argument
generic.
diff --git a/include/proto/server.h b/include/proto/server.h
index 99af0d4..872503c 100644
--- a/include/proto/server.h
+++ b/include/proto/server.h
@@ -105,11 +105,12 @@
 					       const char *weight_str);
 
 /*
- * Parses addr_str and configures sv accordingly.
+ * Parses addr_str and configures sv accordingly. updater precise
+ * the source of the change in the associated message log.
  * Returns NULL on success, error message string otherwise.
  */
 const char *server_parse_addr_change_request(struct server *sv,
-                                             const char *addr_str);
+                                             const char *addr_str, const char *updater);
 
 /*
  * Return true if the server has a zero user-weight, meaning it's in draining