lnpeer: make process_message async

[SomberNight]

This allows making any message handler async in lnpeer.

Note: process_message is only called from _message_loop.
There are(would be) basically three types of message handlers:

1. "traditional blocking msg handlers". non-async ones. When these handlers are called, process_message naturally blocks until the handler returns, which means _message_loop also blocks until the message is fully processed before starting the next iteration.
2. "async blocking msg handlers". async ones where we want the previous property, i.e. we want the _message_loop to wait until the handler finishes. We await the handler inside process_message, and _message_loop awaits process_message.
3. "async non-blocking msg handlers". async message handlers that can be spawned e.g. onto Peer.taskgroup and the loop is free to start processing subsequent messages. e.g. msg handlers that start a negotiation, such as on_shutdown and on_open_channel.

Any non-async message handler (def on_...) automatically goes into category 1.
An async message handler, by default, goes into category 2, "blocking";
to go into category 3 ("non-blocking"), we use the runs_in_taskgroup function decorator.