eventhubs: minor errata with the Producer

[richardpark-msft]

• ProducerClient allows you to set the maximum message size at the client level. In Go, the only place a message size can be set is when creating a batch. I believe that's the case, across the board.
• submit_batch should be renamed, as we generally use 'send' as the verb. So perhaps either send_batch(), or send_event_batch()?
• batch.len() and batch.size() were confusing to me when looking through EventBatch's methods. What do you think about renaming size() to size_in_bytes(), or similar?