Fixed STM deadlock when calling crawl with numThreads <= 0

We should fork at least one thread so that anything happens. It would be
nice if I could enforce this with a dedicated >0 type, but alas doing
this is a bit cumbersome so let's just do a quick fix.
Added small performance concern

The response of httpLbs is not lazy; the entire message will be loaded
in memory; this may be overkill for our purposes.