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.)

The binary format of quality stores one over *Quality* (the ratio of output to input). So smaller binary values are better for the taker. At some point we should stop using naked integers to represent quality and use this class instead:
https://github.com/ripple/rippled/blob/develop/src/ripple/app/book/Quality.h
It abstracts the binary format so that callers don't have to worry about the inverted meaning.