Leader MPM:
This is an experimental variant of the standard worker MPM.
It uses a Leader/Followers design pattern to coordinate work among threads:
http://deuce.doc.wustl.edu/doc/pspdfs/lf.pdf

As of Apache 2.0.39, the key learnings from the development of the
leader MPMs have been applied to the worker MPM.  Worker now is
running faster than leader in benchmark testing.
