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!
Seems likely it should be floating point.  if `request["fee_mult_max"].asInt()` has typical C++ behavior, then the multiplier is rounded down during conversion from float to int.  I don't think we want our fees rounded down.