Reduce load on GET /subscribers endpoint

[chris13524]

This endpoint is slow due to downloading all subscribers in 1 go, and many dapps poll this endpoint frequently. This will become a problem as the list becomes large, and more dapps are doing it. We need some alternatives:

• An endpoint to check if a specific account is subscribed: Get subscribers endpoint #352
• Webhooks to send subscribers to a dapp when they are subscribed, rather than needing to poll the endpoint
• Broadcast functionality to send a notification to all subscribers of a notification type
• Broadcast groups to allow dapps to update the group periodically and broadcast to that, rather than needing to supply the full list in every request

These features should be prioritized based on the load on the service, and what alternatives will fulfill the dapp's needs.

Download the full list of subscribers is still a valid use case, and it's uncertain if there is benefit into implementing API pagination into this specific query, but to continue the function of this endpoint we need to:

• Inform dapps to use an alternative above
• Implement database streaming for get subscribers endpoint #308
• Add bandwidth limiting or prioritization for the download (if possible)
• Add rate limiting (e.g. 1 per hour)