Moved connection keep-alive decision responsibility from HttpAsyncRequestExecutor to individual HttpAsyncClientExchangeHandlers

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1457536 13f79535-47bb-0310-9956-ffa450edef68
Use external SocketAddressResolver strategy to resolve connection route to a local / remote address

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1457537 13f79535-47bb-0310-9956-ffa450edef68
Deprecated ExecutionContext in favor of HttpCoreContext

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1457539 13f79535-47bb-0310-9956-ffa450edef68