From: Nick Piggin <piggin@cyberone.com.au>

Sometimes a processes thinktime shouldn't be measured on how soon it
submits its next request, but how soon any close request is submitted.

Some processes, such as those in make -j, find | xargs blah, etc. Should be
waited upon even if they never submit another request, because they work with
cooperating or child processes. This helps to take that into account.