Re-add a minimal version of BoltDB for migrations

If we're going to maintain migration capability for the full
lifespan of 6 - and I think we're going to have to - the only
sane options are a separate binary that exclusively performs
migrations, or re-adding BoltDB code - in a very minimal way - to
allow us to perform migrations within the standard 6 binary.
After attempting the separate binary approach, results are not
promising - it's impossible to strip enough out to make a truly
small binary that still does what we need to perform a migration.
That leaves re-adding BoltDB code.

This adds a minimal version of the BoltDB code that no longer
claims to be a valid State (freeing us from the requirement of
continued maintenance - we should never touch these bits again
until they get removed in 7) which has just enough to get every
container, pod, and volume in the DB, so we can migrate them to
SQLite.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>