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.)

This is a great write up. Could you clarify a bit about how the "covering" path is different than the other paths?