event: Refactor async event processing

- Improve the implementation of deferred/immediate events.
- Use the new queue module to change how/when events are queued/processed by
  giving a private queue to each emitter.
- Immediate events(which only exist to break uv_run recursion) are now
  represented in the `loop->fast_events` queue.
- Events pushed to child queues are propagated to the event loop main queue and
  processed as K_EVENT keys.

src/nvim/main.c

@@ -157,11 +157,11 @@ void event_init(void)
 
 void event_teardown(void)
 {
-  if (!loop.deferred_events) {
+  if (!loop.events) {
     return;
   }
 
-  loop_process_all_events(&loop);
+  queue_process_events(loop.events);
   input_stop();
   channel_teardown();
   process_teardown(&loop);