refactor to increase code maintainability - move configuration of connections into connection pool classs

Background:

file lib/gitlab/mail_room.rb contains

          if gitlab_redis.sentinels?
            config[:sentinels] = gitlab_redis.sentinels
          end

This code is too tightly coupled to an implementation detail of the pubsub functionality and should be moved into the connection pool management rather than its client.

What questions are you trying to answer?

increase long term maintainability of code

What do you already know about the areas you are exploring?

What does success look like at the end of the project?

Code to configure connections to sentinels should be moved into the connection pool classes.

Links / references:

technical debt