Check for a bot_shutdown even after the MP termination task is completed
Check for a bot_shutdown even after the MP termination task is completed

Fixes race condition where termination task has completed but bot hasn't posted bot_shutdown event. A bot deleted at this moment will reappear when it posts the bot_shutdown.

BUG=589984

Review-Url: https://codereview.chromium.org/2445483002

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