Clean up Pathfinder.

* 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.
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.
Fix account_lines, account_offers and book_offers result (RIPD-682):
The RPC account_lines and account_offers commands respond with the correct ledger info. account_offers, account_lines and book_offers allow admins unlimited size on the limit param. Specifying a negative value on limit clamps to the minimum value allowed. Incorrect types for limit are correctly reported in the result.
Set version to 0.26.4-sp1