refactor attestation code to be more async ergonomic

[jordanhendricks]

@hawkw had some good feedback on #1091 that we punted on for the sake of urgency. In particular, a couple of things about the attestation server are a bit awkward:

• JoinHandle on the server is a leftover from a previous design and could be removed (mvp vm attestation #1091 (comment))
• we could probably make the VM conf into a tokio::sync::watch channel (mvp vm attestation #1091 (comment))

Both of these would be nice improvements to the code and seem worth doing.