Lightning: Add new notifications for plugins

[darosior]

A feedback from a plugin developer on IRC.

• A channel_closed notification would be nice to have.
• A htlc_accepted notification would be nice too (I interpreted as this specific user wanted to monitor the payloads and didn't need to customize, as I understood, lightningd behaviour so a hook was unnecessary).

Could I have the feature request flag please ?.. :-) EDIT: Thanks !

[cdecker]

Added a couple of tags as requested :-)

I think this might be a good first issue for new contributors since it can be implemented using existing infrastructure.

I wanted to say that the hook can be used like a notification by returning {"result": "continue"}, however notifications can be dispatched in parallel, while hooks are called sequentially, so there's definitely a good reason to add the hook as well.

[fiatjaf]

Having the reason for the channel closure somehow would be great (mutual from us, mutual from them, unilateral because of fees, unilateral because of expiring HTLCs, or other reasons).