Worker: Hack around another multiprocessing hang bug
Worker: Hack around another multiprocessing hang bug

Apparently, if you attempt to join on a worker, and the worker tries
the send a message to you through a pipe, the worker blocks inside
the send.  To fix this, we now wait for acknowledgement after sending
quit messages.

mikest/geode,omco/geode,mikest/geode,omco/geode,mikest/geode,mikest/geode,omco/geode,omco/geode