Detecting RAM pressure on nodes

[eduble]

It could be interesting to detect RAM pressure on nodes and in this case send a WALT log line.
https://docs.kernel.org/accounting/psi.html

If using this technique of polling /proc/pressure/memory with POLLPRI, we cannot do that in the busybox-based walt-init script, so we can only do that in the walt-node package with python. It would then be available on images which include this optional package. But this is the case for most images.

This would be most interesting for virtual nodes, which have a quite low default RAM amount.