Fix limit of number of functor cache compilation jobs

It seems that !1909 (merged) (accidentally) increased the limit of number of functor cache compilation jobs to 25. Since each takes a lot of RAM (6-7 GB), on machines with less physical memory (lxplus, online plus) the OOM killer kicks in and it's not pretty.

This MR attempts to configure a smart limit such that the number of jobs used scales with the available physical memory.