FM-360: List snapshots

[aakoshh]

Closes consensus-shipyard/ipc#181

• Snapshot manifest type which we can store along the parts in a manifest.json
• Export the manifest
• List snapshots in a directory
• Initialise the existing snapshots and add them to STM state
• Add new snapshot manifests to the STM state
• List snapshots from the STM data structure in the ABCI
• Integration test that tests a snapshot being created and added to STM

[adlrocha]

Are snapshots only read when starting up the peer? If this is the case I think it is OK to panic.

[aakoshh]

Here you are looking at the code from @cryptoAtwill that streams the blockstore DAG. The stream either returns a pair of Some CID and content, or None to show that it's finished, but there is no way to signal errors.

There was a problem with it (I can't remember now how it manifested itself exactly - something about EOF) when I tried to export the data that the genesis put in place, because the Wasm files are aren't actual IPLD data structures, so the .unwrap() at line 223 panicked. I think that every encoded by DAG_CBOR was placed by ourselves in this part of the code, so it should be okay to panic, it should parse.

To answer your question: this isn't happening when we start the peer, this is happening at regular intervals when we are exporting the snapshots from the store.