Start commands directly in correct cgroups

Gitaly is currently first launching commands, and then adding them to the correct cgroup. This has the downside that the commands get to execute without the appropriate limits being applied for some time before Gitaly manages to configure the Cgroup. This commit changes Gitaly to launch commands directly in the correct cgroup. This gets rid of the brief window with incorrect limits, and should also be more efficient than moving processes between cgroups.