Snapshot support

[penberg]

Currently, the replicated Raft log grows unbounded. First, we need to add Raft snapshot support to Little Raft, which allows truncating the log. We then need to add snapshot support to ChiselStore with, for example, SQLite online backups: https://www.sqlite.org/backup.html. IOW, normal reads and writes could go to an in-memory database, and at snapshot time, an on-disk backup is created. When a node is restarted, the on-disk backup could be read to an in-memory database.