ensure_bootstrap: bootstrap tools in parallel

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>

ensure_bootstrap

@@ -40,14 +40,17 @@ source "$base_dir/cipd_bin_setup.sh"
 cipd_bin_setup
 
 # Sync "gsutil.py".
-python "$base_dir/gsutil.py" -- version 1> /dev/null
+python "$base_dir/gsutil.py" -- version 1> /dev/null &
 
 # Sync all the pylint versions.
 for script in "$base_dir"/pylint-[0-9].[0-9]; do
   # We have to silence stderr too because newer pylint versions will emit
   # a spurious log to tell us what pylintrc file it's using.  Ugh.
-  "$script" --version >/dev/null 2>&1
+  "$script" --version >/dev/null 2>&1 &
 done
 
+# Run bootstraps in parallel to help speed things up.
+wait
+
 # Cleanup.
 find "$base_dir" -iname "*.pyc" -delete || true