feature request: Checkpoint quality of life improvements

Long story short is it would be super handy to checkpoint to an archive file and to write multiple checkpoint spans to different destinations at the same time.

I'm using checkpoints to snapshot key ranges in pebble and it'd essentially be super nifty to be able to write the SSTables directly to an archive file when doing that, sort of like passing a VFS to pebble I was imaging.

Similarly it'd be nifty to be able to checkpoint multiple ranges to different destinations at the same time, so I could split a large pebble database into ranges in one atomic iteration.

Not sure if either would be useful to anyone but myself but thought I'd through it out there!

Jira issue: PEBBLE-1160