Here's one way out of the multi thread collector NPE.

See https://github.com/nathanmarz/storm/wiki/Troubleshooting#nullpointerexception-from-deep-inside-storm

I've tweaked the join bolt's flusher task to only flush when it notices a flush hasn't occured in a while.  This seems to mitigate, but not eliminate, the problem.

In the explaining comment I declare that we don't want to just get & ack exclusively from the flusher thread.  Now I'm not so sure..