parallel_emerge: ensure cleanup runs.
parallel_emerge: ensure cleanup runs.

Specifically, lock down the queue shutdown pathways so they're reusable
and executing the same set of steps.

BUG=chromium-os:19208
TEST=manual verification of code paths
TEST=./setup_board --board=x86-alex
TEST=./build_packages --board=x86-alex

Change-Id: Ib6fab0ab6d6cf401f97a7ffc0619959050c1fc0a
Reviewed-on: https://gerrit.chromium.org/gerrit/20056
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

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