Refactor honk-rpc to use sans-io style interface

[morganava]

We're actually already pretty close to this already. This will make the API more testable and allow for usage with async IO and non-tcp-socket communication channels.

References:

• https://sans-io.readthedocs.io/how-to-sans-io.html
• https://www.firezone.dev/blog/sans-io