Proposal: Sync notifications adapter

[Zetanova]

The current Simpleblob interface for the storage backend can be paired with an Simplenotify interface.

The new interface could abstract away the current S3 storage_poll_interval mechanism
and maybe extended later by a MQTT implementation.

This would make it possible that the producer would publish the URI of the new snapshot to mqtt
and the consuming nodes would use the URI (can be presigned) to get the new snapshot.

[kpfleming]

AWS S3 and MinIO support 'bucket event' notifications already, which can be delivered in a variety of ways. It might be better to leverage that instead of creating a mechanism that risks inconsistency with the actual storage.

[Zetanova]

Storj does not and even witht he bucket event feature from aws or minio, LS does not utilize it.

[Zetanova]

What a dedicated pub-sub call would enable is to use the FileStorage backend.

LN could use the FileStorage backend and to drop the snapshots without any changes to the current implementation into a directory and then generate an URL provided with with an simple template from the config.

The directory could be a public folder of a webserver like apache secured with basic-auth
and the url could include the auth args.
The receiver could then simple use it to directly download the snapshot over a http-get.

[cholcombe973]

I think this is the best way to go because listing S3 every second racks up a lot of cost without much benefit. Bucket notifications are way cheaper and a lot easier on the backend. I don't think it would be difficult to throw together a small PR that does this.