Graceful Shutdown

Hertz supports graceful shutdown, which is executed as follows：

1. Set the state of engine to closed
2. Sequential non-blocking trigger callback function []OnShutDown (consistent with standard library net/http)
3. Shut down the signal listening of the network library
4. Select waits for the business coroutine to exit：
   1. For netpoll network library, turn on ticker with default 1s (set in netpoll, not changeable at the moment) and check if active conn (business handle exits and connection is not in blocking read state) is 0 at regular intervals; for go net network library, turn off listening and do not process the connection.
   2. Triggered by the context of ExitWaitTime, default 5s
5. Uniformly add Connection:Close header to request packets in the process of closing

If you want to modify the wait timeout, you can configure it with server.WithExitWaitTime().

If you want to register the hook function, you can do so by getting the Engine and registering it:

h.Engine.OnShutdown = append(h.Engine.OnShutdown, shutDownFunc)