Running each bootstrap in serial seriously slows things down, and it
gets worse as we add more.  On my very expensive desktop, the no-op
case takes 5 seconds.  When running in parallel, it's now ~1.5 sec.

Change-Id: I4304fddf22ef5a718218a807aeb05e08e5418bb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3245746Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>