Alert on CB processes getting OOM killed

We should flag up that a process has been OOM killed, as this is a common enough problem.

Possible approaches:

1. node_exporter has node_vmstat_oom_kill
2. agent that scans dmesg

In theory there's also the approach of scanning babysitter.log, but that's prone to false positives because OOM killer uses SIGKILL which can be caused by other things (still bad, but shouldn't be labelled as a "OOM kill" when it really isn't)