Azure-webjobs-sdk: Add support for sending service bus messages in batch

[scramsby]

WebJobs supports returning multiple Service Bus Queue messages via ICollector or IAsyncCollector. However, these messages are still submitted via individual Send/SendAsync() calls to QueueClient. Improved performance could be achieved by submitting them via a batch using SendBatch/SendBatchAsync(), subdividing the batches as needed if the batch size exceeds the maximum message size.

See here for an example of how to subdivide batches when they exceed the maximum size:
https://blogs.msdn.microsoft.com/paolos/2015/03/26/how-to-implement-a-partitioned-sendbatch-method-for-azure-service-bus-entities/

[scramsby]

See #570 for the request to support _receiving_ message in batch.

[DarinMacRae]

It's been over a year since this was put on the backlog. Can anyone give us an update on this being added to the SDK, or a more current alternative, or do we need to roll this ourselves (e.g. NamespaceManager.CreateFromConnectionString(connectionString).GetQueue("myqueue"))?

[natelaff]

There were commits for this, but did it actually get fixed? Even running 3.x it still sends these one by one?