Improve alerting mechanism

[xsreality]

Currently, all alerts are sent by looping through list of subscribers in a consumer poll loop and firing alerts if

1. User subscription is active.
2. User's preferred state matches the update or...
3. User has no preferred state

As number of subscribers is growing quicker than anticipated, this flow is not scalable. Instead this can be done via Kafka streams by repartitioning and joining the subscribers topic with the alerts topic. The resulting stream can be stored in another topic which will be picked up by the bot to fire alerts.