Reverse Iterators should 'skip to previous' when start key doesn't exist

[rvagg]

LevelDB only does 'skip to next' when start key doesn't exist but when you want to iterate in reverse you want your start point to be the previous closest key where your actual start doesn't exist.
We can detect and handle this situation for reverse Iterators. Edge case is end-of-database which will cause !Valid().

[kesla]

Sounds like fun. I'm interested in fixing this. Give me a day or two and you should see a PR, if that works for you.

[rvagg]

Cool. Work from 0.2-wip. It's pretty much ready for merging to master and releasing.