search : Extend "intial burst" optimization to return all results by chunks

This way, the user gets a steady (but bursty) stream of reults. We
double the chunk size each time since each successive chunk has to
redo work from all previous chunks.

Of course, the overall time is thereby slower, as the price we pay for
increased responsiveness. With a search returning about 17000 thread
results I measured a total time of 48.8 seconds before this change and
58.4 seconds afterwards.