Add helper method to Transport for adding outbound handlers: Transport.add_outbound_handler
Fix vas2nets transport tests failing after removing Transport._process_message (these could use the test case's worker helper's dispatch_outbound)
Don't use a lambda when adding the default outbound message handling for Transport, subclass Transport in tests instead
Rename kw param for Transport.add_outbound_handler: endpoint -> endpoint_name
Make `msgs.append` an outbound handler in Transport tests instead of needing a toy subclass that overrides handle_outbound_message
Use Transport._send_failure instead of a deeply nested errback in Transport._make_message_processor
Transport._send_failure -> Transport._send_failure_eb
Merge branch 'feature/issue-742-add-helper-method-to-transport-for-adding-outbound-handlers' into develop