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/buffer.c

@@ -70,7 +70,6 @@ String buffer_get_line(Buffer buffer, Integer index, Error *err)
 /// @param line The new line.
 /// @param[out] err Details of an error that may have occurred
 void buffer_set_line(Buffer buffer, Integer index, String line, Error *err)
-  FUNC_ATTR_DEFERRED
 {
   Object l = STRING_OBJ(line);
   Array array = {.items = &l, .size = 1};
@@ -83,7 +82,6 @@ void buffer_set_line(Buffer buffer, Integer index, String line, Error *err)
 /// @param index The line index
 /// @param[out] err Details of an error that may have occurred
 void buffer_del_line(Buffer buffer, Integer index, Error *err)
-  FUNC_ATTR_DEFERRED
 {
   Array array = ARRAY_DICT_INIT;
   buffer_set_line_slice(buffer, index, index, true, true, array, err);
@@ -171,7 +169,6 @@ void buffer_set_line_slice(Buffer buffer,
                       Boolean include_end,
                       ArrayOf(String) replacement,
                       Error *err)
-  FUNC_ATTR_DEFERRED
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
 
@@ -339,7 +336,6 @@ Object buffer_get_var(Buffer buffer, String name, Error *err)
 /// @param[out] err Details of an error that may have occurred
 /// @return The old value
 Object buffer_set_var(Buffer buffer, String name, Object value, Error *err)
-  FUNC_ATTR_DEFERRED
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
 
@@ -375,7 +371,6 @@ Object buffer_get_option(Buffer buffer, String name, Error *err)
 /// @param value The option value
 /// @param[out] err Details of an error that may have occurred
 void buffer_set_option(Buffer buffer, String name, Object value, Error *err)
-  FUNC_ATTR_DEFERRED
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
 
@@ -426,7 +421,6 @@ String buffer_get_name(Buffer buffer, Error *err)
 /// @param name The buffer name
 /// @param[out] err Details of an error that may have occurred
 void buffer_set_name(Buffer buffer, String name, Error *err)
-  FUNC_ATTR_DEFERRED
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
 
@@ -472,7 +466,6 @@ void buffer_insert(Buffer buffer,
                    Integer lnum,
                    ArrayOf(String) lines,
                    Error *err)
-  FUNC_ATTR_DEFERRED
 {
   buffer_set_line_slice(buffer, lnum, lnum, false, true, lines, err);
 }