timer_task: add schedule_one()

gateway/src/resty/concurrent/timer_task.lua

@@ -89,6 +89,13 @@ function _M:execute(run_now)
   run_periodic(run_now or false, self.id, self.task, self.args, self.interval)
 end
 
+--- Schedule one task.
+function _M:schedule_one(delay)
+  -- Need to wrap the task in a function that discards the first param (the
+  -- "premature" one sent by ngx.timer.at)
+  ngx.timer.at(delay or 0, function(_, ...) self.task(...) end, unpack(self.args))
+end
+
 function _M:cancel()
   _M.unregister_task(self.id)
 end