threading: Fix a race around frame completion (frame-mt)

The completion of the first frame to decode while an async reset
request on that same frame is pending will render it stale. The
processing of such a stale request is likely to result in a hang.

One reason this happens is the skip condition at the beginning of
reset_task_cur().
=> Consume the async request before that check.

Another reason is several threads producing async reset requests in
parallel: an async request for the first frame could cascade through the
other threads (other frames) during completion of that frame, meaning
not being caught by the last synchronous reset_task_cur() after
signaling the main thread and before releasing the lock.
=> To solve this we need to add protections at the racy locations. That
means after we increase first, before returning from
reset_task_cur_async(), and after consuming the async request.