Upload artifacts asynchronously in ArchiveStage.
Upload artifacts asynchronously in ArchiveStage.

Archive stage now uploads in parallel with other tasks, and
uploads up to 16 files at a time. This should significantly
speed the time required to run archive stage, which is currently
the bottleneck for our builds.

BUG=chromium-os:23357
TEST=Run trybot run, testing uploads in parallel.
     Unit tests. Pylint.

Change-Id: Ibb0c5d4b7a9024ec51da495b6d19da376822a066
Reviewed-on: https://gerrit.chromium.org/gerrit/12039
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,coreos/chromite,coreos/chromite,coreos/chromite,zhang0137/chromite