Remove "actor are stopping when they detect there are no more messages" functionality

[fulmicoton]

Right now Actor stop as soon as they detect no one will ever send them a message.

This is done by checking the refcount over their mailbox. Once it reaches 1, it means only their copy (in the actor context) exists and no new message will come.

While this sounds neat, it implies a lot of papercuts to enforce.

For instance,

• The actor handle does not expose the actor mailbox directly. (That would break our refcount mechanism)
• Therefore spawning returns (Mailbox, ActorHandle)
• Self scheduling breaks the logic, therefore we still need to send a termination message.

The suggestion is to remove this functionality and force the chain of actor to send a quit command downstream.

[fulmicoton]

Relying on explicit messages would also have the benefit of not identifying the death of upstream actors as a success and make the pipeline supervision simpler.

Right now a successful pipeline means all actors exited with a success. We could change that to the publisher exited with a success (which is not valid semantic today because the packagers panicking would lead to that too).

That would solve the problem of how do we stop the GC considering it keeps running and prevents reaching the "all actors exited successfully" condition.

[fulmicoton]

Moving to Quickwit 0.5. Also we still need ot check whether this is a good idea or not.