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.)

Quality is a good term and I think that having it represent the ratio of output over input is workable, with the understanding that the binary format is the inverse. This is perfectly reflected in the `Quality` class.