removed HackMpm and MaxWrite config directives, added dynamic write size...

removed HackMpm and MaxWrite config directives, added dynamic write size behaviour according to recommendations from Ilya Grigorik similar to Apache Traffic Server

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1695920 13f79535-47bb-0310-9956-ffa450edef68