Cleanups to RPC code.

This is extracted from a significantly longer RPC change that I put down a couple of weeks ago.

It deletes some unused code, uses Issue and Book a little more, and fixes some long lines and indentation.

Feel free to use this or ignore it!
This is a bit of a hack designed to make it easier for a developer to add an RPC handler strictly on their own fork of rippled, and without getting significant merge conflicts.