boot: use several processes to bootstrap on Win32

[emillon]

This workaround does not seem to be necessary (anymore?); the mechanism with several commands work, including when concurrency = 1.

[emillon]

@dra27 you wrote this bootstrap code; do you remember why it was useful to have windows-specific behavior here?
(context is #9507 and we're wondering if we should remove that code path).

I'll also try to find a way to benchmark how much memory that saves.

[dra27]

I have a vague recollection it was sufficiently faster to justify the extra code path

[dra27]

Indeed - it seemed to be just empirical evidence (see #2854) - if it’s not the case any more, then marvellous!

[emillon]

I ran the numbers:

OS	strategy	time
linux	single	33.59
linux	j1	41.11
linux	j2	26.76
linux	j3	21.36
linux	j4	19.55
windows	single	380.29
windows	j1	302.25
windows	j2	280.99
windows	j3	323.05
windows	j4	346.51

Bootstrap times for linux

Bootstrap times for windows

I agree that there's something weird going on with Windows, and it's probably pretty noisy (I only took one measurement here) but all in all it doesn't that the separate code path is worth it.

(I tried to benchmark memory usage, but did not fine any difference either using cygwin's /usr/bin/time nor with this approach using powershell).

[emillon]

Closed in favor of #9735.