neo: "Operation is not valid due to the current state of the object" error when consensus node handle 512 txs

Describe the bug when handling 512 transactions to generating a new block, consensus service will stuck and throw “Operation is not valid due to the current state of the object” error.

To Reproduce Steps to reproduce the behavior: neo-cli: master 51cd29fbe21abb9e1f17f64e5c6d21bc7decbbb9 neo: master ab4830cae3242e80fd60e7df6c0d9143803b7be3 neo-vm: master be2ac36bf35a3033d828e0ba0630d390599c487d MillisecondsPerBlock is 15000.

1. build latest neo-cli with neo/neo-vm project.
2. deploy 4 consensus node.
3. deploy 1 external node
4. send 50 tx/sec and last about40secs to external node.

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

Expected behavior consensus nodes are expected to handle massive txs.

Screenshots

It can be seen from the screenshot that when number of txs is 216, consensus will continue, and when number of txs is 512, then error shows.

[18:17:18.052] initialize: height=35 view=0 index=3 role=Primary
[18:17:33.058] timeout: height=35 view=0
[18:17:33.059] send prepare request: height=35 view=0
[18:17:33.189] OnPrepareResponseReceived: height=35 view=0 index=0
[18:17:33.299] OnPrepareResponseReceived: height=35 view=0 index=1
[18:17:33.302] send commit
[18:17:33.880] OnCommitReceived: height=35 view=0 index=1 nc=1 nf=0
[18:17:34.118] OnPrepareResponseReceived: height=35 view=0 index=2
[18:17:34.124] OnCommitReceived: height=35 view=0 index=2 nc=2 nf=0
[18:17:34.137] relay block: height=35 hash=0xa5d4b10cfaf0c6b24f598c975235b15d6399176fb4b0fb1fd41224d43a250103 tx=216
[18:17:34.164] persist block: height=35 hash=0xa5d4b10cfaf0c6b24f598c975235b15d6399176fb4b0fb1fd41224d43a250103 tx=216
[18:17:34.165] initialize: height=36 view=0 index=3 role=Backup
[18:17:49.159] OnPrepareRequestReceived: height=36 view=0 index=0 tx=512
[18:17:49.164] send prepare response
[18:17:49.890] OnPrepareResponseReceived: height=36 view=0 index=1
[18:17:49.892] send commit
[18:17:49.910] OnCommitReceived: height=36 view=0 index=1 nc=1 nf=0
[18:17:50.145] OnCommitReceived: height=36 view=0 index=0 nc=2 nf=0
[18:17:50.150] relay block: height=36 hash=0xb3be2ffab0fd2ac9f46dd8d14e5727d9d6c3f7e739cf72cf2c034d2100cc40cb tx=512
[ERROR][1/14/2020 10:17:50 AM][Thread 0005][akka://NeoSystem/user/$a] Operation is not valid due to the current state of the object.
Cause: System.InvalidOperationException: Operation is not valid due to the current state of the object.
   at Neo.Ledger.Blockchain.Persist(Block block) in D:\NEO\Github\neo\src\neo\Ledger\Blockchain.cs:line 503
   at Neo.Ledger.Blockchain.OnNewBlock(Block block) in D:\NEO\Github\neo\src\neo\Ledger\Blockchain.cs:line 333
   at Neo.Ledger.Blockchain.OnReceive(Object message) in D:\NEO\Github\neo\src\neo\Ledger\Blockchain.cs:line 477
   at Akka.Actor.UntypedActor.Receive(Object message)
   at Akka.Actor.ActorBase.AroundReceive(Receive receive, Object message)
   at Akka.Actor.ActorCell.ReceiveMessage(Object message)
   at Akka.Actor.ActorCell.Invoke(Envelope envelope)
[ERROR][1/14/2020 10:17:50 AM][Thread 0027][akka://NeoSystem/user/$a] Error while creating actor instance of type Neo.Ledger.Blockchain with 2 args: (Neo.NeoSystem,Neo.Plugins.Storage.Store)
Cause: [akka://NeoSystem/user/$a#456841214]: Akka.Actor.PostRestartException: Exception post restart (System.InvalidOperationException)
 ---> System.TypeLoadException: Error while creating actor instance of type Neo.Ledger.Blockchain with 2 args: (Neo.NeoSystem,Neo.Plugins.Storage.Store)
 ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> System.InvalidOperationException: Operation is not valid due to the current state of the object.
   at Neo.Ledger.Blockchain..ctor(NeoSystem system, IStore store) in D:\NEO\Github\neo\src\neo\Ledger\Blockchain.cs:line 107
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture)
   at Akka.Actor.Props.ActivatorProducer.Produce()
   at Akka.Actor.Props.NewActor()
   --- End of inner exception stack trace ---
   at Akka.Actor.Props.NewActor()
   at Akka.Actor.ActorCell.CreateNewActorInstance()
   at Akka.Actor.ActorCell.<>c__DisplayClass109_0.<NewActor>b__0()
   at Akka.Actor.ActorCell.UseThreadContext(Action action)
   at Akka.Actor.ActorCell.NewActor()
   at Akka.Actor.ActorCell.FinishRecreate(Exception cause, ActorBase failedActor)
   --- End of inner exception stack trace ---

Platform:

• 4 consensus nodes: n1 - Ubuntu 18.04 n2 - CentOS 7.4 n3 - Winserver 2016 n4 - Ubuntu 18.04 external node - Ubuntu 18.04