Experiment with BoltDB buckets

[roman-khimov]

We can use BoltDB buckets instead of key prefixes simplifying key creation and likely improving performance. But this will require us to change the generic DB interface (Store) adding prefixes explicitly to delete/get/put/seek operations, so the main question is --- is it worth the trouble? Only BoltDB supports that, so other DBs will still have to play with prefixes. We need to try it out with DB restore and neo-bench and then make some decision.

[roman-khimov]

An easy way to try it out is to strip out the prefix in the DB backend.

[roman-khimov]

See #1748.