Remove the usage of custom header X-BatchID

We wrongly used a message header X-BatchID to identify the batch of a message in some of our API. Message are identified by the Message-ID header, and the usage of that custom header is an implementation details that should not leak in messages. Moreover, not all mime message will contain this header, causing spurious API failure (implicit hidden API).

So, we should expose explicitly the batchId in some API that are currently using the custom header as a required information, and remove the custom header from sent mails.