Dynamically set db_pool if Puma is in use
As mentioned in omnibus-gitlab#4631 (closed), if Puma is in use, we should dynamically set the Rails db_pool to be a function of the number of threads.
As mentioned in omnibus-gitlab#4631 (closed), if Puma is in use, we should dynamically set the Rails db_pool to be a function of the number of threads.