Shadow can use a lot of system resources like file descriptors, memory maps, ram, etc. We should monitor these resources while Shadow is running and log a warning message if we appear to be near the system's limits.

This should not be expensive since we only need to monitor the shadow process and not the plugins, so should only take a few syscalls to get the information we need.

It's undecided where the best place is to check the resource usage, but one option is in _manager_heartbeat() which logs every X simulated seconds.