Shorten default parsed filenames to just the trailing filename parts

This is just a API change for the default key name from Parse.
Add a nthreads parameter to control F/J threads

"-nthreads 99" caps the lowest-priority F/J queue to 99 threads.  This
is the batch queue where all the "real" work gets done; capping this
will limit batch CPU usage.  Making this too low can lead to deadlock.
Merge branch 'master' of github.com:0xdata/h2o