Autocreate database snapshot

[varasev]

We need an ability to create database snapshot automatically without stopping the node. This would allow other users download the snapshot and use it to avoid syncing their nodes from scratch.

The snapshot should be created every N days (or hours) in a form of some binary or archive file.

Probably, the snapshot and restore commands could be used/extended for that: https://openethereum.github.io/CLI-Sub-commands#snapshot