Listen filtering behavior for TLS connections?

[tfpauly]

As I was updating text for #333, I noticed that the Listener object delivers connections, but doesn't provide a particular way to reject connections. Based on existing text, I assume that ReceivedConnection delivers a connection once the TLS server handshake is complete. Some implementation may want to rate limit, or modify/block inbound connections.

Should we have some event that lets applications handle inbound connections before they are ready?

Of course, for TLS, we can rely on callback handlers set for the handshake, but that requires each protocol to do this potentially.

[tfpauly]

@britram mentioned that post-sockets had a notion of an event for pre-accept to be able to reject connections, maybe at multiple levels?

[britram]

As discussed in Montréal:

1. Listeners should have backpressure #343 covers the backpressure case.
2. Each protocol in the stack at listen time may want to ask the application whether or not an incoming request should be allowed to continue. As far as we know, most of the applications that care about rejecting connections before a TLS handshake will do this outside the application.