st/mesa: implement st_server_wait_sync properly

st/mesa: implement st_server_wait_sync properly

Asynchronous flushes require a proper implementation of
st_server_wait_sync, because we could have the following with
threaded Gallium:

 Context 1 app     Context 1 driver         Context 2
 -------------     ----------------         ---------
 f = glFenceSync
 glFlush
 <-- app sync -->                           <-- app sync -->
                                            glWaitSync(f)
                                            .. draw calls ..
                   pipe_context::flush
                     for glFenceSync
                   pipe_context::flush
                     for glFlush

Reviewed-by: Andres Rodriguez <andresx7@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>