Some "minorish" UA changes:

 - User Agent string of Upload Agent is updated to include platform,
   and is also more verbose for /UPLOAD/ requests (includes dxcpp & dxhttp user agent string)
 - Added some sleep() to compress/read/upload threads, to ensure that we yield to other threads
   (Note: boost::this_thread::yield() is not an interruption point, so we couldn't use it)