Chunk Length based keys

I'm putting in this pull request, because I got it working,
but I don't think you should merge it, because it turned out uglier and more complicated
than using monotonic timestamp.

Append can no longer be the default case, because you have to check what the last record saved was, (which is actually quite complicated, because of this issue:  rvagg/node-levelup#110 )

that all means that you have to query the db several times before it can start writing,
and adds more code to the deps.

I will put in another pull request for hashes though.