dynamic pool size with gnumake jobclient / jobserver

[milahu]

use gnumake jobclient to automatically grow and shrink the pool size
to limit the overall cpu load in a build system (make, cmake, ninja, ...)

this is useful on large build servers (example: 128 cores)
but the build system runs with make -j32 to limit cpu load

by default billiard.Pool(1000) and multiprocessing.Pool(1000)
will start 1000 workers *before* demand
when a jobserver is found, pool should start only one worker, and grow the pool *on* demand

for every extra worker request a token from the jobserver.
if the jobserver is full, dont start new workers

example implementation
blink-bindgen-limit-jobs-with-jobclient.patch

this can only grow the pool on demand
ideally it should also shrink the pool when workers run idle
→ release tokens to the jobserver, so other workers can run

challenge:
release tokens on exception or sys.exit
otherwise other workers can hang forever
probably with sys.excepthook

ideally "someone" would write a better jobserver protocol
where the jobserver is actively monitoring the worker processes (could also monitor cpu load)
and when a worker crashes without releasing tokens, the jobserver assumes the tokens are released
could be based on celery or similar