* Allow pathfinding requests where the starting currency may have
multiple issuers.
* Cache paths over all issuers to avoid repeating work.
* Clear the ledger checkpoint in one retry case.
* Add an additional node at the front of paths when the starting issuer
is not the source account.
* Restrict to 80-columns and other style cleanups.
* Make pathfinding a free function and hide the class Pathfinder.
* Split off unrelated utility functions into separate files.
