Race condition on harvester limit

[tsg]

The test_prospector.py:Test.test_harvester_limit test is flaky. The fail can be reproduced by running it in a while loop like this:

 while nosetests test_prospector.py:Test.test_harvester_limit; do echo .; done

On my computer, this fails after 10 or 20 iterations.

Looking at the logs and code, it seems to be a race condition between checking p.harvesters.Len() and p.harvesters.Start(). We could serialise the whole startHarvester() method with a new lock, but I'm slightly worried about dead locks, because the harvester registry has it's own lock. So I'm opening a ticket for now to separate this from #5449.