Make accept of Transport cancel safe

[rapiz1]

This is unlikely to occur. But if a hot-reload for a service and a TCP connection arrives at the same time, the TCP connection will be dropped.

rathole/src/server.rs

Lines 140 to 143 in 1240dd8

	tokio::select! {
	// FIXME: This should be cancel safe.
	// Wait for incoming control and data channels
	ret = self.transport.accept(&l) => {

A primitive thought is to turn Transport::accept into Transport::handshake and make it accept a raw TCP connection. And use TCPListener::accept in select because it is cancel safe. However it breaks some abstraction and assumes the underlying protocol is TCP, which causes trouble for UDP-based transport, like QUIC. So maybe the trait can be redesigned in another way. Thoughts will be welcome.

Or keep the trait unchanged and write a future to make Transport::accept cancel safe. I don't know if that's feasible. For TCP, it's impossible to put an accepted connection back to the system backlog. And generally handshake consists of multiple packets. I doubt it can made cancel safe without altering the trait. Redesigning the trait seems the way to go.