upnp: download xml doc using std::future

The UpnpDownloadXmlDoc is now called using shared_future. The api
function is a blocking call and there were instances where,
depending on the router and wifi speeds, the function might fail
or hang. By using std::shared_future, if such a thing arises, the
program won't block.

Change-Id: Ibca8d926e8980ebd4519f0e6bbd6ce2125396159