chore: restructured node

[jcabrero]

Inspired by @mfontanini PR for the keeper, created this to simplify the structure of the blacklight-node and make it more readable.

• It replaces tokio::Notify with a tokio_utils::cancellation_signal that should tell the node to interrupt the listening task. In this case it was working because there is always a listening for cancellation event and only once concurrent tokio::select! waiting for the cancellation signal. It would have worked with notify_once but I believe it is more solid using a cancellation signal in case the node was ever doing more than one listening event in parallel.
• I created a supervisor module, which runs the main loop and is in charge of client reconnections in case of failures.
• It has some functions to listen for events, and to process htxs. The rest of the functionality remains in the same places.

[jimouris]

Looks good, but didn't check too thoroughly.