Transactions with insufficient funds can be on-chain and lead to consensus service stuck

[cloud8little]

Describe the bug
Transactions with insufficient funds can be on-chain and lead to consensus service stuck.

To Reproduce
Steps to reproduce the behavior:

1. Run one seed node.
2. Run a single consensus node, connect to the seed node
3. Run an external node.
4. Withdraw the initial NEO/GAS and send 100k GAS to a sender wallet.
5. Open the sender wallet, use (sendfrom)rpc request to send about 800k requests, 0.00000001 GAS/request to another account.
6. Around 1hour later, consensus service stuck, not generating blocks anymore.

when the consensus service stuck, there is only 459+GAS with the account.

Expected behavior
Those transactions with not enough gas check should be discarded.
Consensus service should not stuck.

Screenshots

Platform:

• OS: Ubuntu18.04
• Version
  neo-cli: master Limit incoming connections based on param. Support connecting to trusted IPs. #470
  neo: master Add NeoAPI and update code after testing with NEO3 preview1 #1150 + pr1220

[Tommo-L]

This problem related to #1140 @vncoelho

I think we can allow this situation happen but change the calculation of sys_fee.

neo/neo/SmartContract/Native/Tokens/GasToken.cs

Lines 37 to 53 in 72be7f6

	protected override bool OnPersist(ApplicationEngine engine)
	{
	if (!base.OnPersist(engine)) return false;
	foreach (Transaction tx in engine.Snapshot.PersistingBlock.Transactions)
	Burn(engine, tx.Sender, tx.SystemFee + tx.NetworkFee);
	ECPoint[] validators = NEO.GetNextBlockValidators(engine.Snapshot);
	UInt160 primary = Contract.CreateSignatureRedeemScript(validators[engine.Snapshot.PersistingBlock.ConsensusData.PrimaryIndex]).ToScriptHash();
	Mint(engine, primary, engine.Snapshot.PersistingBlock.Transactions.Sum(p => p.NetworkFee));
	BigInteger sys_fee = GetSysFeeAmount(engine.Snapshot, engine.Snapshot.PersistingBlock.Index - 1) + engine.Snapshot.PersistingBlock.Transactions.Sum(p => p.SystemFee);
	StorageKey key = CreateStorageKey(Prefix_SystemFeeAmount, BitConverter.GetBytes(engine.Snapshot.PersistingBlock.Index));
	engine.Snapshot.Storages.Add(key, new StorageItem
	{
	Value = sys_fee.ToByteArray(),
	IsConstant = true
	});
	return true;
	}