A lot of new things have happened in Messenger for 4.3. This is a meta issue to try to track then:

• New PHP Serializerintroducing native php serialize() support for Messenger transport symfony#29958
• LoggingMiddleware is deprecated[Messenger] deprecate LoggingMiddleware in favor of providing a logger to SendMessageMiddleware symfony#30539
• new event dispatched when sending a messageDispatching two events when a message is sent & handled symfony#30650
• Better output when you runmessenger:consume[Messenger] add welcome notice when running the command symfony#29303
• Command was renamed/shortened tomessenger:consume Done in[Messenger] Rename the messenger:consume-messages command #11262
• MessageBus::dispatch() accepts Envelopes are 2nd arg[Messenger][DX] Allow stamps to be passed directly to MessageBusInterface::dispatch() symfony#30707
• Envelope 2nd constructor arg changed[Messenger][DX] Allow stamps to be passed directly to MessageBusInterface::dispatch() symfony#30707
• Command to setup transports[Messenger] Add a command to setup transports symfony#29476
• Retry support -[Messenger] Worker events + global retry functionality symfony#30557
• ReceiverInterface methods changed -[Messenger] Worker events + global retry functionality symfony#30557 and[Messenger] ReceiverInterface::handle() to get() & Worker with prioritized transports symfony#30708 - also a few behavioral things changed inside them
• UnrecoverableMessageHandlingException can be thrown in handler to fail and avoid retry -[Messenger] Worker events + global retry functionality symfony#30557
• Ability to dispatch a message from within a message and have it handled in a separate Doctrine transaction[Messenger] Support for handling messages after current bus is finished symfony#28849
• prefetching optionAdd optional parameterprefetching for AMQP connection symfony#30671
• New messenger:stop-workers Command[Messenger] New messenger:stop-workers Command symfony#30754
• New way to see size of messages in a transport[Messenger] Adding MessageCountAwareInterface to get transport message count symfony#30757
• New sync transport[Messenger] Adding the "sync" transport to call handlers synchronously symfony#30759
• Failed message handling[Messenger] Adding failure transport support symfony#30970
• multiple queues support for amqp with routing key -[messenger] Adds a stamp to provide a routing key on message publishing symfony#30008
• Allows to register handlers on a specific transport[Messenger] Allows to register handlers on a specific transport symfony#30958
• support for handling multiple transports in one worker in a prioritized way. See this comment and attached PR:[Messenger] ReceiverInterface::handle() to get() & Worker with prioritized transports symfony#30708 (comment)
• RoutableMessageBus route to default bus[Messenger] RoutableMessageBus route to default bus symfony#31288
• Doctrine ping connection middleware[BridgeDoctrineMessenger] Doctrine ping connection middleware symfony#31061
• Add WorkerStoppedEvent[Messenger] Add WorkerStoppedEvent symfony#31282
• New middleware (not important unless we're documenting all of them somewhere) -[Messenger] On failure retry, make message appear received from original sender symfony#31425 (there are also other new middleware, which hopefully are mentioned here)
• send_and_handle messenger routing got removed in favor of SyncTransport[Messenger] remove send_and_handle which can be achieved with SyncTransport symfony#31454