get peer ID upon handler creation

[drHuangMHT]

fn new_handler(&mut self) in trait NetworkBehaviour does not provide the peer which the newly created handler is dedicated to. The only way to inform the handler of the peer itself is connected to seems to be emit an event to "request" for peer ID after the handler was created. Is that by design or can be improved potentially by adding another argument to new_handler()? When you try to notify a non-exist handler the event will be dropped silently, which will, in my case, cause an oneshot sender be dropped resulting in error on the receiving end, that's not expected.

[thomaseizinger]

Currently, this can be achieved by implementing IntoConnectionHandler instead.

Once #3254 merges, it will be even easier.

[drHuangMHT]

I see. That's fantastic!