An implementation of bakery.RootKeyStore that uses boltdb as a persistent store.
This implementation uses the dbrootkeystore.RootKeys of the bakery for generating and caching chunks by implementing the dbrootkeystore.Backing interface with boltb as a backend. To improve performance, the database uses two buckets. The keyBucket stores root keys by id and the seqBucket one stores the key ids using an auto-incremented index. For that reason it is assumed that keys are always inserted in the same order as they are created.