[Sentinel/Keva] Pub/sub for Sentinel

Sentinels that monitor the same Redis instance discovers each other by subscribing to the channel "__sentinel__:hello" of that Redis instance, and publish its own information to this channel each 1 second, so that other sentinels know about it.

Keva does not have Pub/sub yet, but Sentinels need an intermediary to communicate.

At first, i think we can workaround by including sentinels information in Redis INFO command result, and sentinel can get that when it periodically make call to the instance.
And another internal endpoint for Sentinels to introduce itself to Redis instances.

No files or tests are identified. Start by tracing the existing Redis INFO command and Sentinel integration, then evaluate the proposed Sentinel information endpoint and INFO-based discovery; done means Sentinels monitoring the same instance can introduce themselves and discover one another without native pub/sub.

Written by the indexing model from the issue text.