Fix race condition in OutboxState

[sebastianburckhardt]

Fixes a problem discovered during testing.

While iterating over the collection of messages to send, even if this is just a single message, the send can actually complete, and the send confirmation can also complete, at which point the element may be removed from the collection, which causes a conflict with the initial loop iteration as we see here:

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at DurableTask.Netherite.OutboxState.Send(Batch batch) in D:\git\durabletask-netherite\src\DurableTask.Netherite\PartitionState\OutboxState.cs:line 112
   at DurableTask.Netherite.DurabilityListeners.ConfirmDurable(Event evt) in D:\git\durabletask-netherite\src\DurableTask.Netherite\Util\DurabilityListeners.cs:line 79
   at DurableTask.Netherite.Faster.LogWorker.Process(IList`1 batch) in D:\git\durabletask-netherite\src\DurableTask.Netherite\StorageLayer\Faster\LogWorker.cs:line 168

This PR fixes the problem by copying the collection into an array before iterating over it.

[davidmrdavid]

Some nits and questions

[davidmrdavid]

hmm, what was this concurrent mod about? Not sure how this count-check is related to concurrent modifications

[sebastianburckhardt]

This logic was based on the assumption that concurrent modification is only a problem if the list is larger than one element. That assumption was wrong. So we now always make a copy of the list.

[sebastianburckhardt]

Thanks for the review. I have committed your suggestions.