Actors discard all exsting messages in the mailbox while shutdown

[contrun]

We use ActorCell::stop_children_and_wait to stop all children actors in https://github.com/contrun/fiber/blob/8978c15308106ff5b93df9ef0479ac5b6c628fa6/src/actors.rs#L53-L55. The problem of this is that we will run post_stop function of the actor and discard all messages in the mailbox. So it is possible the channel actor successfully processed the CommitmentSigned from a peer, and then sent a command to network actor to send a RevokeAndAck message, but this command was placed in the mailbox and discarded while shutting down.

It is a nice-to-have that we try some efforts to process existing messages in the actor mailboxes while shutting down. As far as I can tell, ractor does not have any infrastructure to accomplish this easily.