Missing `CompleteBatch` in Java library

The ServiceBus java library is missing the `CompleteBatch` method that [exists in the C# SerbviceBus library](https://docs.microsoft.com/en-us/dotnet/api/microsoft.servicebus.messaging.messagereceiver.completebatch?view=azure-dotnet).

For processes that depend on receiving messages from the service bus, this is a **major** performance bottleneck since completions require a request for every single message, **significantly** impacting performance. This is especially strange since `receiveBatch` is supported. So processes can receive many messages in a single request, but completing all of them requires individual requests.

We did some benchmarking on this, and it seems that even the premium service bus instances (at 690$/Month/MU) are capable of supporting only a few tens of thousands of requests a minute. Because a request is required per message, there are tens of thousands of requests. So the `completeBatch` is a crucial in order to reduce the number of requests.