upnp: fix intermittent xml download error

Depending on the platform, libupnp sometimes fails to download
the xml document provided by devices that answer the search
request. re-enabling blocking the blocking tcp connections at
compile time for libupnp seems to fix this issue.

The function used by the libupnp api to download the xml documents
is purely synchronous. To prevent the function from blocking
the upnp thread in case it hangs in the api, we used the disable
blocking tcp connection flag. Now that std::future is used to
circumvent this problem, we can re-enable the blocking tcp
connection.

Change-Id: I6efa02521e25171fcbca8c58a2c93987f5414f29