sstate: show progress bar again

Transition to ThreadPoolExecutor (eb6a6820)
broke the

| Checking sstate mirror object availability: ...

progress bar because the removed 'thread_worker' was still referenced
in an asynchronous function.  As the result of the future is never
read, the resulting backtrace is silently discarded.

Replace the information given to 'ProcessProgress' by a counter.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>