This adds O(reviewers+cc) RPC to Gerrit before upload,
though done concurrently in up to 10 threads. This information isn't
used yet, but will be in follow up CLs.

R=ehmaldonado

Bug: 877717
Change-Id: I2374f249ee874a71089244309e50e8e88a3dee7d
Reviewed-on: https://chromium-review.googlesource.com/c/1242847Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>