State export & import

[cwgoes]

We need the ability to export and import state to and from a genesis file (collection of TOML files). This may be useful in case Namada needs to perform an irregular state-modifying upgrade where specific data structures kept in state must be manually modified or where the keyspace layout needs to change. This requires:

• each validity predicate can export its state to and import its state from part of the genesis file (e.g. MASP - commitments, nullifiers, notes, etc.; governance - proposals, delegates, etc.)
• the ledger can export its state to and import its state from part of the genesis file (e.g. replay protection, other system-level key/value state pairs)

The export and import functions should be inverses; i.e., when we export state and reimport it on a different node, we get exactly the same state. This can be tested by hashing the entire storage, exporting it, importing it, re-hashing the storage on the new node, and checking that the hash matches the original. It's important that we're able to take the state export and start up a new chain (with the export as genesis), we should test this flow as well.

We should be able to use Serde for JSON serialization & deserialization, so this shouldn't require too much more than paired import / export functions for all VPs and other parts of state, where:

• the import function takes a bunch of structs (deserialized from JSON) and writes them to the appropriate DB keys
• the export function reads all relevant DB keys into structs and serializes them to JSON (which can be imported)

We'll also need a namada export <filename> command (no special import command should be required, just the usual genesis start).

This work may have conflicts with ongoing genesis flow refactors by @batconjurer; we should communicate with him.

[adrianbrink]

We should be able to test this on a testnet quite easily.

Run a chain with 4 validators and 2 nodes. Let the chain run for 1000 blocks. Stop 1 validator and 1 node. The chain continues. Export the state on 1 validator and 1 node. Clean the data directories of the 1 validator and 1 node. Import the state into the validator and node. Both should continue where they had exported it.

[cwgoes]

Agreed with @sug0 to start with a simpler version that does not serialize/deserialize everything to JSON, but just add an import DB command to mirror the dump DB command, and write a simple demo of exporting & importing state, including performing some simple modification.

[cwgoes]

Done by #2092 #2835. Needs to be tested.