Excises this btrfs namespacing crap.

This also object-disorients lib/btrfs. Reducing it to a simple set of
functions. I'm pretty sure it should stay that way. It will help
perpetuate the "all state is expressed in the filesystem" ethos.
Merge pull request #14 from pachyderm-io/simplify_btrfs

Excises this btrfs namespacing crap.