Removed unnecessary plugin cnfiguration

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1629596 13f79535-47bb-0310-9956-ffa450edef68
Refactored the default and pipelining async exchange handlers

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1629597 13f79535-47bb-0310-9956-ffa450edef68