input.c: Process only safe events before blocking.

Introduce multiqueue_process_priority() to process only events at or
above a certain priority.

src/nvim/api/vim.c

@@ -714,7 +714,6 @@ Dictionary nvim_get_mode(void)
   Dictionary rv = ARRAY_DICT_INIT;
   char *modestr = get_mode();
   bool blocked = input_blocking();
-  ILOG("blocked=%d", blocked);
 
   PUT(rv, "mode", STRING_OBJ(cstr_as_string(modestr)));
   PUT(rv, "blocking", BOOLEAN_OBJ(blocked));