Shrink bolt databases

[fyrchik]

The size of bbolt databases can only grow boltdb/bolt#308

This is a bit misleading from the user POV, who can do df -h and also leaves less space for big objects.
We can extend GC with a vacuum, which (rarely) tries to compact blobovnicza trees by trying to move all objects on a single level to a smaller set of leaves.

There is also https://github.com/etcd-io/bbolt/blob/master/compact.go, but it needs 2 files.