core: stateDb has no trie and no snap return err

[weiihann]

Description

This PR adds a check to Blockchain.StateAt() such that if the node is a fast node (i.e. NoTrie = true) and the snap layer cannot be found, then it will return an error.

Rationale

Refer to #2331.

By default, a fast node will have an empty trie for any state. Retrieving a value from an empty trie will give 0x00...00. It may be confusing for users as 0x00...00 could also be the actual value. Hence, it's more useful to throw an error when the state is not available.

Example

Command

curl -H 'Content-Type: application/json'  -X POST --data '{"jsonrpc":"2.0", "method": "eth_getStorageAt", "params": ["0x00020F14443e103A0B72F1f2DFDaac07DDAa6Df5", "0x0", "0x237f199"], "id": 1}' http://127.0.0.1:8545

Output

{"jsonrpc":"2.0","id":1,"error":{"code":-32000,"message":"state is not available"}}

Changes

• Add StateDB.GetSnap()

[flywukong]

LGTM