Moved JSONRPCRequest to its own header

Summary: Need to move this class for upcoming RPC diffs in addition to locating it in its own separate header like it should be.

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1588