Notifications

[JssDWt]

Add notifications when the peer is offline.

• grpc server for registering a notification url for your pubkey.
• Sending a notification if the peer is offline and a registration was present.
• Hold a htlc a (configurable) little while when a notification was sent, waiting for the peer to come online.

This is a very basic implementation of #63. It does not include the following features yet:

• Removing a webhook subscription after expiry or because it's not functioning or because the client wants to removee the webhook subscription.
• Log of published webhooks
• Retry mechanism for temporary failure to deliver the webhook call.
• Initial subscribed notification
• Chain notifications
• Solid mechanisms for not publishing duplicate notifications after restarts or replay of htlcs.

Note that the time period for holding the htlc might get tricky when you get close to the cltv delta. So the holding period should generally be short.

[roeierez]

@JssDWt I think this is great! I dropped some comments to discuss.

[roeierez]

LGTM

[JssDWt]

• added another integration test
• fixed a bunch of bugs related to checking whether the peer is online and looking up the peer based on the scid
• renamed updated_at to refreshed_at
• also wait for the channel to become active after the peer comes online if the htlc is a regular forward

More efficient peer online and scid lookups are out of scope for this PR and is tracked by this issue: #77

[JssDWt]

Merged into this PR more efficient lookups of peer id based on scid and more efficient waiting for the peer to come online (that was blocking this PR to be merged) #78

[JssDWt]

Closing this in favor of #88