sync: switch to multiprocessing.Event

We've switched most of this command over to multiprocessing and off
of _threading, so do the Event object too.  The APIs are the same
between the modules, so we shouldn't need to update anything else.

Change-Id: I52d31f1c6ef2bcbe7bbc1dd1add79a8d5d08784a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305147
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>