WIP: Make Redis connection pool configurable

One can now set a Redis connection pool size via gitlab.rb.

By default, this should be left unset; instead, we let the application choose an appropriate size at runtime based on the concurrency of the current execution environment (see gitlab!19778 (closed)). However, should the user specify a custom value in /etc/gitlab/gitlab.rb, which should pick this up and override any dynamic values set at runtime (although we may decide to set a cap; TBD.)