[p2p] Support Self-Sending (?)

[patrick-ogrady]

Add support for a channel wrapper that allows self-sending or natively incorporate into p2p. In many cryptographic applications, there is a need to message to "self" (holding multiple roles) and this approach can make it easier to implement mechanisms as-is: #2354

[patrick-ogrady]

This didn't end up being necessary for #2439 and suggest we probably avoid this (seems unnecessary and/or error-prone to burden p2p with loopback). Will keep thinking.