Refactored client context handling in CachingHttpAsyncClient

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1572562 13f79535-47bb-0310-9956-ffa450edef68
Upgraded HttpClient dependency to version 4.3.3

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1572563 13f79535-47bb-0310-9956-ffa450edef68