Return peak memory usage in activation response JSON

Problem: there's no easy way to find out if an action is consistently getting close to hitting the memory limits (as opposed to time limits, which is clearly apparent)

It would be very useful if the activation result json could show the peak memory usage so you can see how close it is to hitting the limits (eg, peak_memory_usage: 486/512 MB)

From @rabbah in Slack:

> this is doable; the information should be stored in the cgroup eg `/sys/fs/cgroup/memory/docker//memory.max_usage_in_bytes`