Renamed RPCServer namespace to RPCServerSignals

Summary: Needed so I can repurpose RPCServer as a new class name. RPCServerSignals is more explicit anyway.

Test Plan: make check and build/test/functional/test_runner.py

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1583