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.)

Yeah, that's exactly what you wrote, isn't it? 
> In addition, if no selected path is capable of providing enough liquidity to complete the payment by itself, an extra "covering" path is returned.

That sentence could be rewritten to change the emphasis, or that concrete example could be included, or I could read more carefully in the future. :)