Fix pathfinding with multiple issuers for one currency (RIPD-618).

* 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.
Remove unused, confusing isXRP(Issue) function.

* Not used in codebase.
* Whether an Issue "is XRP" isn't well-defined.