Add HTTPRPCRequestProcessor to own httprpc

Summary: Adding HTTPRPCRequestProcessor provides an easy way for Config and any future (RPCServer) references to be passed around between HTTP and RPC servers.

Test Plan:
make check && test_runner.py
Also tested RPC directly:
bitcoind -regtest
bitcoin-cli -regtest getexcessiveblock
It works!

Reviewers: deadalnix, schancel, Mengerian, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1736