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!
@rec So, this use-case favors an RPC handler implementation that uses a common abstract base over std::function objects (because the Manager won't need to see the declaration of the handler).