Official Fulcrum RPA Server Support

The proposal: Add official support to Fulcrum for the RPA (Reusable Payment Address) privacy feature.

This privacy feature is specified here: https://github.com/imaginaryusername/Reusable_specs/blob/master/reusable_addresses.md

It has been implemented in the past by Jt here: https://github.com/Electron-Cash/Fulcrum-RPA

It has been updated and synched to latest Fulcrum master, but it is still slow and needs more work: https://github.com/fyookball/Fulcrum

I want to officially support it in Fulcrum, as the Fulcrum author, and further optimize it to make it production-ready!