Remove the unused function WelsSleep

Windows Phone 8 doesn't have Sleep(), but there's no need to
use the function at all.
Use more modern versions of functions that don't exist on Windows Phone
Use the Windows Runtime ThreadPool API for creating threads on Windows Phone

Windows Phone lacks the old CreateThread/beginthreadex APIs for
creating threads. (Technically, the functions still do exist,
but they aren't officially supported and aren't visible in the
headers when targeting Windows Phone.)

Building code that uses the Windows Runtime language extensions
requires building with the -ZW option.
Merge pull request #610 from mstorsjo/wp8-threads

Make threading work on windows phone
Merge branch 'master' of https://github.com/cisco/openh264