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.
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