Configure `Redis::Wrapper.pool_size`

Today, we seem to hard-code some number of Redis connections in application-code: https://gitlab.com/gitlab-org/gitlab/blob/master/lib%2Fgitlab%2Fredis%2Fwrapper.rb#L24.

This should be either configurable, or be adapted to support Puma concurrency.

This means that every Puma configuration with over T5 might be starved out of Redis connections.

We might as well extend Omnibus to expose this config.