Fix message balancing between replicas with Redis Sentinel

[Grumov]

The current distribution of messages between replicas is uneven. The replica following the master in the list of servers is selected more often than others. In the configuration there is a master and two replicas:

• the first replica receives 66% of messages
• the second replica receives 34%.

[NickCraver]

@Grumov Rather than jumping to a PR here that creates a lot of other issues, why don't we start with an issue on what your problem is? This jumps to making connection changes in critical paths and changing the server selection for every command sent through the library. That's quite a leap to make, let's back up to an issue and start with what problem you're trying to solve.

[NickCraver]

I see we do have an issue #2554, awesome - let's discuss the issue there and figure out how to proceed.