Always empty the pmap output queue.
Always empty the pmap output queue.

Otherwise stack traces references can be stuck in the output queue, causing
significant use of memory for no reason.

Logs the exceptions that are discarded when there's more than one exception.

R=stip@chromium.org
BUG=chromium:525873

Review URL: https://codereview.chromium.org/1413693011

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py