Support concurrent processing of messages by actor

[njam]

As far as I understand the current implementation of start_actor() will process messages sequentially. The next message will only be processed, once the previous one is completely processed, even if the actor's async fn handle() yields.

Theoretically async fn handle() could be executed concurrently instead, for example by using something like StreamExt#for_each_concurrent().
But then handle() cannot be given mutable access to self anymore obviously. So the actor would need to use interior mutability to change state.

What are you thoughts on this?