Parallel verification issues

[shargon]

Describe the bug
While reviewing the bug #1410 I used multiple threads for the rpc request and I advice that it produce a similar denial of services that is described in #1410

The error was produced here

neo/src/neo/Ledger/Blockchain.cs

Line 500 in fc1a64b

if (engine.Execute() != VMState.HALT) throw new InvalidOperationException();

Because a double spend in Gas native token

neo/src/neo/SmartContract/Native/Tokens/Nep5Token.cs

Line 100 in fc1a64b

if (state.Balance < amount) throw new InvalidOperationException();

To Reproduce
Steps to reproduce the behavior:

1. Create a 1 node network
2. install rpc module
3. open wallet by rpc
4. send multiple sends with at least 5 threads

{
  "jsonrpc": "2.0",
  "method": "sendtoaddress",
  "params": ["0x8c23f196d8a1bfd103a9dcb1f9ccf0c611377d3b", "NNa1Fgc82Qoh65TJfViqC92W8Z8AWL9YKA",0.00000001],
  "id": 1
}

Expected behavior
Consensus nodes are expected to handle massive txs.

Additional context
Possible fix #1417

[Tommo-L]

I think this is the reason, I'll test it.

[eryeer]

That make sense, we need to recheck the balance after parallelVerify.

[shargon]

@Tommo-L If it's scenario, it's not related to parallel verification because TX1 and TX2 can enter in mempool with balance, and later transfer his funds to other address, when they enter in the block, the second one can produce the same behaviour.

[Tommo-L]

No, it's related to parallel verification.

If the transaction will transfer his funds to other address, it'll update the mempool after persisted by Mempool.UpdatePoolForBlockPersisted