External event discarded when the durable function is in Pending state

**Description**
After a long period of inactivity, if we start a new instance of the durable function the initial status is _OrchestrationRuntimeStatus.Pending_ and after 2 seconds the status is updated to _Running_.

If we send and external event to the durable function, invoking the _IDurableOrchestrationClient.RaiseEventAsync()_, while the status is Pending, sometimes the event is discarded!

**Expected behavior**
We expect that the event is not discarded while the status is pending.

**Actual behavior**
As from the application insights the event is dequeued and discarded because the instance of the durable function not exists ("No such instance") but it's not true.
.
After the RaiseEventAsync(), our application log a message with the status of the durable fuction that was _Pending_ (_"Orchestrator '0cbedce2-61b8-41fc-84bf-697d31b80a10' status:'Pending'."_) .

**App Details**
Durable Functions extension version: 2.6.0
Azure Functions runtime version: 4.0.1
Programming language used: .NET (C#)

まず、IDurableOrchestrationClient.RaiseEventAsync() が使用する外部イベントのパスと、OrchestrationRuntimeStatus.Pending のインスタンスをどのように処理するかを調査します。Durable Functions extension 2.6.0 と Azure Functions runtime 4.0.1 で説明されているタイミングを再現し、その後、Pending 中に発生させたイベントが "No such instance" としてデキューされず、オーケストレーションが Running に到達した時点で利用可能であることを確認します。

索引モデルが issue の本文から書いたものです。