Better resource utilization through load average monitoring

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1333)
In GitLab by [[Gitlab user @lle-bout]](https://gitlab.com/lle-bout) on Jun 1, 2020, 07:15

GNU make has `-l` for limiting number of jobs according to load average. See: https://www.gnu.org/software/make/manual/html_node/Parallel.html

Gentoo's emerge has `--load-average` with a similar effect. See: https://wiki.gentoo.org/wiki/EMERGE_DEFAULT_OPTS

I am thinking Buildstream could take advantage of such an option to spawn more build jobs in parallel until load average reached a configurable value.

It has the advantage of being quite trivial to implement without any changes or manual tagging of build recipes. In short, an easy performance win.

No BuildStream file, test, or entry point is named. Begin by locating the build-job scheduling and configuration entry points, then compare the proposed behavior with GNU make's -l and Gentoo's --load-average documentation. Done means an agreed configurable load-average policy, implementation scope, and tests for spawning jobs up to that limit.

Written by the indexing model from the issue text.