Added RPCServer and defined its API, wrapping the old implementation

Summary: Completes T401. Prep work for T387. Depends on D1736.

Test Plan: make check && test_runner.py

Reviewers: deadalnix, schancel, Mengerian, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1721