Capsules and Port Reuse

[ehaydenr]

With the introduction of capsules, a proxy receives request headers and client connection id separately. In order to reuse ports, a proxy must know the client connection id when selecting a socket so that it can properly demultiplex when receiving on that target-facing socket. We should clarify what clients and proxies should do to increase the likelihood of proxies being able to reuse target-facing ports.