Make get-rss less expensive

[grembo]

Is your feature request related to a problem? Please describe.
nomad-pot-driver calls "pot get-rss" once per second per container. When running many containers, this causes a lot of forking.

Example on a host running 6 containers:

# while sleep 1; do ps -ax | grep -i "ps -ax"; done
73060  0  R+       0:00.00 ps -ax
73061  0  S+       0:00.00 grep -i ps -ax
73303  0  R+       0:00.00 ps -ax
73304  0  S+       0:00.00 grep -i ps -ax
73546  0  R+       0:00.00 ps -ax
73547  0  S+       0:00.00 grep -i ps -ax
73789  0  R+       0:00.00 ps -ax
73790  0  S+       0:00.00 grep -i ps -ax
74032  0  R+       0:00.00 ps -ax
74033  0  S+       0:00.00 grep -i ps -ax

Describe the feature you'd like to have
Not entirely sure how to approach this best - doing some actual profiling with very cheap containers would be a first step.

One idea would be some sort of daemon that writes rss information of all processes controlled by a pot instance to a file that can be used as a source to make "pot get-rss" cheaper? In general this feels like it should be solved by reducing the amount of script work involved (maybe potnet could be extended to include more tools that optimize certain things and/or make them more secure, or improve inside of nomad-pot-driver). Keeping as much as possible in shell is a good thing though in terms of iterating over the code and being able to fix things in production easily.

Describe potential alternatives or workaround you've considered (if any)
As a first step, reduce the amount of forking required when calling "pot get-rss".