RFE: start jobs depending on loadavg

[sam-s]

Often I want to start many independent processes on a shared server, and I cannot know in advance how many CPUs will be available. I want to be able to start new process whenever, say, getloadavg1 < 1.5*cpu_count (as opposed to a fixed max_workers ). cf. https://github.com/joblib/joblib/issues/1006 and https://bugs.python.org/issue39617