h2load: Start thread execution using conditional variable
h2load: Start thread execution using conditional variable

When thread is created, we pause them.  After all threads are created,
master thread sends signal to all worker threads and let them start to
benchmark.  This will make thread start almost at the same time since
we can avoid thread creation overhead.  It also exclude thread
creating time from benchmark time.  We also simplified thread creation
routine, and now we always use dedicted worker thread to issue
requests even if -t1.

wzyboy/nghttp2,icing/nghttp2,wzyboy/nghttp2,wzyboy/nghttp2,wzyboy/nghttp2,icing/nghttp2,wzyboy/nghttp2,wzyboy/nghttp2,wzyboy/nghttp2,icing/nghttp2,icing/nghttp2,icing/nghttp2,icing/nghttp2,icing/nghttp2