net: use an interface class rather than signals for message processing

Summary:
Drop boost signals in favor of a stateful class. This will allow the message
processing loop to actually move to net_processing in a future step.

This is part of core PR10756

Depends on D1744 and D1748

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1749