Notifications improvements

[JssDWt]

Taken from #63 (comment)

• maintain a log of published notifications (for diagnostics/support)
• mechanism to remove webhook subscriptions by expiry and/or when the webhook call fails
• retry mechanism is useful when interacting with third party services (so webhook data needs an identifier for deduplication?) could be useful to send an initial 'subscribed' notification when ready.
• the time when waiting for the channel to be active is interesting, especially regarding restarts of lspd/the underlying lightning node.