CSHARP-2287: RetryWrites=true in a transaction throws exception when using async writes.

Author: rstam

src/MongoDB.Driver.Core/Core/Operations/RetryableWriteOperationExecutor.cs

@@ -86,7 +86,7 @@ public async static Task<TResult> ExecuteAsync<TResult>(IRetryableWriteOperation
 
         public static async Task<TResult> ExecuteAsync<TResult>(IRetryableWriteOperation<TResult> operation, RetryableWriteContext context, CancellationToken cancellationToken)
         {
-            if (!context.RetryRequested || !AreRetryableWritesSupported(context.Channel.ConnectionDescription))
+            if (!context.RetryRequested || !AreRetryableWritesSupported(context.Channel.ConnectionDescription) || context.Binding.Session.IsInTransaction)
             {
                 return await operation.ExecuteAttemptAsync(context, 1, null, cancellationToken).ConfigureAwait(false);
             }