Config allows users to select if they want to commit to the shutdown public key or not, but either way we pull the shutdown public key form users at channel-open (storing it in `Channel::shutdown_pubkey`). We should instead actually pull the public key at shutdown-time based on the config bool. https://docs.rs/lightning/0.0.99/lightning/util/config/struct.ChannelConfig.html#structfield.commit_upfront_shutdown_pubkey