Tweak jemalloc to get a better memory usage

It seems that memory usage didn't improve that much after deploying jemalloc to production (https://gitlab.com/gitlab-com/infrastructure/issues/418#note_15668044 and https://gitlab.com/gitlab-com/infrastructure/issues/418#note_15461342).

We need to see if we need to tweak jemalloc to have a better usage; things like number of arenas and such.