[MM-42347] Fix UDP conn not respecting deadlines

[streamer45]

Summary

PR fixes an issue with the UDP server connection not respecting the defined deadlines and hence potentially getting stuck indefinitely in case of an unreachable host.

After some testing and looking into the standard library I've figure out this was actually caused by the call on File.Fd() we do here:

mattermost-plugin-calls/server/activate.go

Lines 55 to 64 in 4ac8eb9

	writeBufSize, err := syscall.GetsockoptInt(int(connFile.Fd()), syscall.SOL_SOCKET, syscall.SO_SNDBUF)
	if err != nil {
	p.LogError(err.Error())
	return err
	}
	readBufSize, err := syscall.GetsockoptInt(int(connFile.Fd()), syscall.SOL_SOCKET, syscall.SO_RCVBUF)
	if err != nil {
	p.LogError(err.Error())
	return err
	}

As suggested in the official documentation we are now using File.SyscallConn() instead which seems to avoid this problem.

Ticket Link

https://mattermost.atlassian.net/browse/MM-42347

[streamer45]

@agnivade This is strictly related to the issue I mentioned to you a while back.

[cpoile]

Whoa, nice debugging!

[agnivade]

Yep saw it, nice work!