Rename CRPCCommand -> ContextFreeRPCCommand

Summary: As part of the RPC refactor to support passing a template manager to the new mining API, I'll be introducing a new RPCCommand class. Renaming the old class to ContextFreeRPCCommand to indicate that passing context is infeasible, given the design.

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1558