notmuch search: Return first 100 results as quickly as possible.

This is one of those cases where total time is not the metric of
interest. We increase the total time of the search, (by doing some
redundant work for the initial threads). But more significantly, we
give the user *some* results nearly instantaneously, (so that the user
might see the result of interest without ever even waiting for the
complete results to come in).