Raft snapshot

Discussed in scope of "RFC - Storage Schema" on google docs.

The aim of this issue is to implement raft snapshot transfer.

Th approximate plan is the following:

1. Snapshot is everything stored in _pico_* spaces.
2. Serialize it and send as is.
3. Check how raft-rs handles large snapshot size and fix if necessary.

Relevant places in code:

• src/traft/node.rs:844 - Snapshot applying
• src/traft/raft_storage.rs:288 - Snapshot generation