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
Revert pathfinding changes:
* 5e7c527 Revert "Fix account_lines, account_offers and book_offers result (RIPD-682):"
* b3417ca Revert "Fix pathfinding with multiple issuers for one currency (RIPD-618)."
* 00db7f5 Revert "Clean up Pathfinder."
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-sp2
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 bugs in pathfinding with XRP as the source currency
Set version to 0.26.4-sp3