msgpack: Replace FUNC_ATTR_DEFERRED by FUNC_ATTR_ASYNC

API functions exposed via msgpack-rpc now fall into two categories:

- async functions, which are executed as soon as the request is parsed
- sync functions, which are invoked in nvim main loop when processing the
  `K_EVENT special key

Only a few functions which can be safely executed in any context are marked as
async.

src/nvim/api/tabpage.c

@@ -62,7 +62,6 @@ Object tabpage_get_var(Tabpage tabpage, String name, Error *err)
 /// @param[out] err Details of an error that may have occurred
 /// @return The tab page handle
 Object tabpage_set_var(Tabpage tabpage, String name, Object value, Error *err)
-  FUNC_ATTR_DEFERRED
 {
   tabpage_T *tab = find_tab_by_handle(tabpage, err);