Limit the number of PVF workers (#4273)

* Limit the number of PVF workers

In particular, limit the number of preparation workers to 1 (soft &
hard) and limit the number of execution workers to 2.

The reason why we are doing this is that it seems many workers launched
at the same time can cause problems. I.e. if there are more than 2
preparation workers, the time for preparation rises significantly to the
point of reaching the timeout.

This was mostly observed with parallel_compilation=true, so each worker
used `numcpu` threads and now we are looking to flip that parameter to
`false`. That said, we want to err on the safe side here and gradually
enable it later if our measurements show that we can do that safely.

* Adjust the test to accomodate the changed config value
20 jobs for master in 47 minutes and 24 seconds (queued for 8 seconds)