Initial implementation.
Add some more test helpers.
Fix .get_batch_key().
Tests for new message storing middlewares.
Remove get_batch_key in favour of get_latest_batch_key.
Remove stray additions.
Fix error message text in .get_batch_id() NotImplementedError.
Comment on why MessageMetadataHelper is imported inside a method.
Merge branch 'feature/issue-599-conversation-and-router-message-storing-middleware' into develop