improve performance of reading sandbox stats (oom, peak memory)

[syphar]

after each command in the build we see (and need) this:

most exec cat calls as <100 ms, but I regularly see calls between 2 and 10 seconds. Which is quite a waste, when we see that we currently do 4 calls after each container command (two for oom, two for memory peak).

First optimization was that we remember which cgroup version we have, so then we can save one call in the later checks.

Then I discovered that we also can get the same information directly in special files from the host. So I added this. From what I see, the locations likely speficic to the used setup / engine, so I left the exec cat approach in. I checked that the "fast" way would be used by docs.rs.

I don't like the added complexity, but the performance impact will likely be noticeable.

While we're on it I extracted the bigger cgroup stats code into a new module, container lifetime stuff stays in Container

Generally: I think most randomly show things are i/o bound, and increasing the bandwidth / iops will hopefully help.

[GuillaumeGomez]

Instead of implementing all that, why not using crates with this feature like sysinfo?