HTTPCLIENT-1298: Add AsynchronousValidator in HttpClientBuilder's list of closeable objects
Contributed by Martin Meinhold <mmeinhold at atlassian.com>

* Resolve constructor dependency between CachingExec and AsynchronousValidator
* AsynchronousValidator implements Closeable interface

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1437952 13f79535-47bb-0310-9956-ffa450edef68