[Service Bus] API review changes to the Batch object and its related methods

[ramya-rao-a]

Findings for the Batch object and its related methods from internal API review for version 7.0.0-b7 of the Azure Service Bus package:

• Rename BatchMessage to MessageBatch. Consider adding ServiceBus prefix after discussing with the Python team
  - If we are adding the ServiceBus prefix, also consider doing so for Message
• max_size_in_bytes on the batch object should be readonly. Log an issue to add this in Event Hubs as well
• message on the batch object need not be public.
• Consider renaming create_batch() method to create_message_batch(). We use the "message"/"messages" term in all other methods on the sender and receiver, so this should sit right well.
• Similarly, consider renaming add() method on the batch to addMessage() to resolve the confusion we saw in certain user studies where it was not clear what the next step was after creating a batch

[ramya-rao-a]

@annatisch Can you provide some help to the points above that we are "considering"?

[annatisch]

Thanks @ramya-rao-a - I'm good with these changes :)