Support limited compaction for versioned files

[ecton]

One of the limitations of compacting files is that all of the version information is removed. Back when we had a few data accidents on CouchDB, this caused some of the mistakes to be unrecoverable -- ie, the customer had to go fix their stuff rather than us save the day.

I'd like to have a way to compact databases while retaining the most recent X versions of each document, or the most recent X writes in addition to a minimum of the latest revision per each key.