Three missing features from GNU Parallel: `--delay`, `--load`, `--memfree`

[dpc]

Describe your issue

We're using GNU Parallel to run a large test suite, and oftentimes when hitting problems with it (like MacOS support glitches) I wish I could move to something more modern. But due to it being a somewhat heavy e2e test suite the tests in it have some deadlines etc. under which we expect things to happen. They are large enough that normally it is not a problem, but it requires that the tests don't pile all up at the same, even though they are not only CPU-bound workloads (they are bursty in nature, use a lot of IO and memory as well as CPU).

GNU Parallel has these 3 features that make it work seamlessly:

--load helps keep the load on the machine within acceptable bounds, irrespective how powerful the dev machine is, and what else they might be using it for,
--memfree makes sure that there is enough memory,
--delay helps stagger the test start so each has some time to fully kick in before parallel makes a decision if the machine has enough compute power to start another one.

[doronbehar]

Not particularly important for running a test suite, but I'd like to add 2 more features I miss in rust-parallel: --tmux and --tmux-pane.