Failover on shutdown for standalone

[zuiderkwast]

Follow-up of #2292, #1091.

We discussed in the weekly meeting to open a follow up for standalone.

Originally posted by @madolson in #2292 (comment)

[gmbnomis]

The FAILOVER option to the SHUTDOWN command would be a perfect addition to sentinel's support for FAILOVER:

The Bitnami Valkey Helm chart in sentinel mode needs to implement graceful failover & shutdown on rolling upgrades. Currently, the logic is quite complex as sentinel is not able to use FAILOVER.

For 9.0, I added the functionality to use FAILOVER to sentinel. But it turns out that many client libraries do not implement the sentinel client protocol fully and their connection handling is racy (they reconnect to the demoted primary and just assume that it is still a primary).

If sentinel used "SHUTDOWN FAILOVER", we wouldn't unpause the clients after failover (I presume) and that race would not happen.