forall: improve pool logic

Use a pool contextmanager to take care of the messy details like
properly cleaning it up when aborting.

Change-Id: I264ebb591c2e67c9a975b6dcc0f14b29cc66a874
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297243


Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>
Tested-by: Mike Frysinger <vapier@google.com>