lexi: Unable to iterate backwards with prefix.

[JoeGruffins]

Using a prefix while iterating should only return results that start with that prefix. However something seems broken when iterating in reverse. This test should work but fails with no results in reverse. JoeGruffins@8f3b015

[buck54321]

I've been hacking at this for a while.

Here's a commit on top of @JoeGruffins commit to show how either badger ain't right, or I'm just not getting it: buck54321/dcrdex@8f3b015...buck54321:dcrdex:reverseprefixduh0

Here's a commit with an adapted fix that was offered for this issue in the badger repo. It works, but then we bomb at a different spot: buck54321/dcrdex@af271ac...buck54321:dcrdex:reverseprefixduh1

I think there's probably a solution to be cobbled together, but damn.

[buck54321]

Not to make things more confusing, but I just tested reverse iteration with a prefix in #3153 (scoredIdx), and it worked fine.