Add dummy serial version UID

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1621222 13f79535-47bb-0310-9956-ffa450edef68
Javadoc8 fixes

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1621225 13f79535-47bb-0310-9956-ffa450edef68
<code> => {@code }

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1621234 13f79535-47bb-0310-9956-ffa450edef68