Allow configuration of batched sender parallelism

src/Wolverine/Configuration/Endpoint.cs

@@ -116,6 +116,11 @@ protected Endpoint(Uri uri, EndpointRole role)
     /// </summary>
     public int MessageBatchSize { get; set; } = 100;
 
+    /// <summary>
+    /// For endpoints that send messages in batches, this governs the maximum number
+    /// of concurrent outgoing batches 
+    /// </summary>
+    public int MessageBatchMaxDegreeOfParallelism { get; set; } = 1;
 
     /// <summary>
     ///     Mark whether or not the receiver for this listener should use

src/Wolverine/Configuration/SubscriberConfiguration.cs

@@ -68,6 +68,16 @@ public T MessageBatchSize(int batchSize)
         add(e => e.MessageBatchSize = batchSize);
         return this.As<T>();
     }
+
+    /// <summary>
+    /// For endpoints that send messages in batches, this governs the maximum number
+    /// of concurrent outgoing batches 
+    /// </summary>
+    public T MessageBatchMaxDegreeOfParallelism(int batchMaxDegreeOfParallelism)
+    {
+        add(e => e.MessageBatchMaxDegreeOfParallelism = batchMaxDegreeOfParallelism);
+        return this.As<T>();
+    }
 
     public T DefaultSerializer(IMessageSerializer serializer)
     {

src/Wolverine/Transports/Sending/BatchedSender.cs

@@ -29,7 +29,7 @@ public BatchedSender(Endpoint destination, ISenderProtocol protocol, Cancellatio
 
         _sender = new ActionBlock<OutgoingMessageBatch>(SendBatchAsync, new ExecutionDataflowBlockOptions
         {
-            MaxDegreeOfParallelism = 1,
+            MaxDegreeOfParallelism = destination.MessageBatchMaxDegreeOfParallelism,
             CancellationToken = _cancellation,
             BoundedCapacity = DataflowBlockOptions.Unbounded
         });