KV + FST backup system

[valeriansaliou]

No description provided.

[valeriansaliou]

• KV: Stream the RocksDB database to a text file, with each binary key as [u8 + u32] numbers and contents as a stream of u32 numbers; do this per-(collection, bucket);
• FST: Stream the FST words to an ordered text file on-disk; this way we can re-import later on to a newer-version FST library (as the FST library may break the on-disk format between versions, this will let us write migration scripts easily); do this per-(collection, bucket);

[valeriansaliou]

Do this over channel protocol; ie. control mode.

[valeriansaliou]

Also add an option to restore a backup (per-collection or per-(collection, bucket) from given path) from the channel in control mode. This would over-write any existing database if needed (KV + FST).

[valeriansaliou]

Benefits of a text-based backup file format is that we generate human-readable backups that can be used for debugging, and that can also be used for easily converting a Sonic binary DB from an older format to a newer format, by bridging via the backup text format.