Added RPCCommand base classes in preparation for RPC migration

Summary:
This is the scaffolding for soon-to-be RPC refactors. RPCCommand enables easy passing of dependencies only to the RPC commands that use them.
New RPCCommands can be written with appropriate dependency injection and old commands can be slowly migrated to use non-global dependencies.

Depends on D2500, D2507

Test Plan: ninja check

Reviewers: deadalnix, #bitcoin_abc, Fabien

Reviewed By: deadalnix, #bitcoin_abc, Fabien

Subscribers: Fabien, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1894