Previously, 1 CL was queried first, followed by all remaining ones
in parallel. The purpose was to ensure that valid authentication
token is present. However, it still required one call to Rietveld
or Gerrit, needlessly increasing latency.

This CL first loops over all CLs and ensures that credentials are
present, refreshing refresh tokens for Rietveld if necessary. Then,
all CLs are queried in parallel.

R=sergiyb@chromium.org,clemensh@chromium.org
BUG=681704

Change-Id: Ic125ac7c2a684d6f3c34e4e8b899192abbed01bb
Reviewed-on: https://chromium-review.googlesource.com/431033Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>