address #279: allow for parallel processing of handshakes

[jordan-bonecutter]

Description

To allow for parallel handshake processing, I propose to introduce the AcceptHandshake method on listener. AcceptHandshake returns a Handshaker interface, which is just a conn who has not yet performed their handshake and a config for doing so. Handshakes may be processed in parallel by calling the Handshake method on the Handshaker in a background goroutine.

Reference issue

Fixes #279