fix stall detection to actually work when fully stalled

fix stall detection to actually work when fully stalled

When fully stalled, the progress bar doesn't update, so waiting on a
MVar would block forever. There's no need to wait anyway, just wake up
after sleeping the configured period and check the current value.

Luckily Viasat makes it really easy for me to notice this kind of
mistake, by stalling long TCP connections frequently.