Small pathfinding fixes (RIPD-735)

Fix a small collection of pathfinding problems explained in RIPD-735. Also, don't process transactions when we're started with --net and have never had a network ledger -- it just produces nonsensical results and we don't relay them anyway.

Reviewers: @vinniefalco, @ximinez

Now with unit tests passing! (There was a bug in determining whether the issuer needed to be removed from the final path output.)

It's designed so that better qualities come first. It's unfortunate that we use the term "quality". Perhaps "cost' would be a better term?