rstream: Rename RStream async flag to defer

The name `async` was not appropriate to describe the behavior enabled by the
flag.

src/nvim/os/rstream.c

@@ -24,7 +24,7 @@ struct rstream {
   uv_file fd;
   rstream_cb cb;
   size_t buffer_size, rpos, wpos, fpos;
-  bool reading, free_handle, async;
+  bool reading, free_handle, defer;
 };
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
@@ -38,21 +38,19 @@ struct rstream {
 ///        for reading with `rstream_read`
 /// @param buffer_size Size in bytes of the internal buffer.
 /// @param data Some state to associate with the `RStream` instance
-/// @param async Flag that specifies if the callback should only be called
-///        outside libuv event loop(When processing async events with
-///        KE_EVENT). Only the RStream instance reading user input should set
-///        this to false
+/// @param defer Flag that specifies if callback invocation should be deferred
+///        to vim main loop(as a KE_EVENT special key)
 /// @return The newly-allocated `RStream` instance
 RStream * rstream_new(rstream_cb cb,
                       size_t buffer_size,
                       void *data,
-                      bool async)
+                      bool defer)
 {
   RStream *rv = xmalloc(sizeof(RStream));
   rv->buffer = xmalloc(buffer_size);
   rv->buffer_size = buffer_size;
   rv->data = data;
-  rv->async = async;
+  rv->defer = defer;
   rv->cb = cb;
   rv->rpos = rv->wpos = rv->fpos = 0;
   rv->stream = NULL;
@@ -333,7 +331,7 @@ static void close_cb(uv_handle_t *handle)
 
 static void emit_read_event(RStream *rstream, bool eof)
 {
-  if (rstream->async) {
+  if (rstream->defer) {
     Event event;
 
     event.type = kEventRStreamData;