Support pool for action_foreach

This is follow up of pool support in action
https://codereview.chromium.org/2926013002

Using action pool can remove some overhead of many running process.
Pool support of action_foreach gives better control for some python generator step when using goma.
e.g. https://codereview.chromium.org/2726103005

Bug: 695864
Change-Id: Ibd0bbaffc59513db42119138520aee3505762eee
Reviewed-on: https://chromium-review.googlesource.com/882625
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Cr-Original-Commit-Position: refs/heads/master@{#531844}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4c48a35125352ab95d8660d2039864ba49f9f7d2