UDP-dispatcher: fix activity timer cause connection disconnected every minutes

[patterniha]

when target-connection is udp, after the first disconnection between xray-client and xray-server, the connection between xray-client and xray-server is disconnected every minute, Let's do the following reproduction-method to understand better:

1. you need python with PySocks package(pip install PySocks)

2. run xray-client and xray-server both on one PC

3. run echo-server.py with python(python echo-server.py)
   this is a simple udp echo server

4. run echo-socks-client.py with python
   this is simple udp echo client that use socks5 protocol to connect

   echo-socks-client.py <-> xray-client <-> xray-server <-> echo-server.py

5. after a few seconds, terminate xray-server and then re-run xray-server (to disconnect connection between client-xray and server-xray, and re-established again)

6. sniff connection between xray-client and xray-server (loopback-adapter-port-1234)

Then you see that every minute the connection between xray-client and xray-server is disconnected and then re-established.(while the connection is not idle and should not be closed)

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this is important issue, that causes us to have frequent disconnections between xray-client and xray-server(after first disconnecting with any reason)

Also, because we bind a new port(freedom-udp) in server-side every time, it can cause a lot of problems.

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Why is this happening?

This is because of this wrong code:

Xray-core/transport/internet/udp/dispatcher.go

Lines 65 to 69 in 1976d02

	removeRay := func() {
	cancel()
	v.RemoveRay()
	}
	timer := signal.CancelAfterInactivity(ctx, removeRay, time.Minute)

After previous-connection is closed, it's timer remains active, and after a minute it run v.RemoveRay() and RemoveRay close v.conn, but now v.conn is our new connection which is active and should not be closed !!!

And this process repeating...

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other refine:

in worker.go it is better to cancel ctx after udpConn is closed