Stop haphazardly passing `InitFeatures` in message handlers

[TheBlueMatt]

handle_{shutdown,open_channel,accept_channel} etc methods all take an InitFeatures parameter because ChannelManager needed it and it was the cheapest way to pass them through. We could pass them through on every message handler, but there's little reason to since ChannelManager caches the last InitFeatures in the peer state anyway, and it would clean up the interface to just remove it.

[TheBlueMatt]

This should be pretty easy to tackle - just remove the InitFeatures parameters from the message handlers, replacing the access to them in ChannelManager with the version stored in per_peer_state.

[danielgranhao]

If no one has yet, I can try to tackle this.

[TheBlueMatt]

Do it!

[TheBlueMatt]

Fixed by #1959