Implementation of the Server Dialback method as defined by the RFC3920

/**

 * Implementation of the Server Dialback method as defined by the RFC3920.

 *

 * The dialback method follows the following logic to validate the remote server:

 * <ol>

 *  <li>The Originating Server establishes a connection to the Receiving Server.</li>

 *  <li>The Originating Server sends a 'key' value over the connection to the Receiving

 *  Server.</li>

 *  <li>The Receiving Server establishes a connection to the Authoritative Server.</li>

 *  <li>The Receiving Server sends the same 'key' value to the Authoritative Server.</li>

 *  <li>The Authoritative Server replies that key is valid or invalid.</li>

 *  <li>The Receiving Server informs the Originating Server whether it is authenticated or

 *  not.</li>

 * </ol>

 *

 * By default a timeout of 20 seconds will be used for reading packets from remote servers. Use

 * the property <b>xmpp.server.read.timeout</b> to change that value. The value should be in

 * milliseconds.

 *

 * @author Gaston Dombiak

 */