BUG/MAJOR: threads/freq_ctr: fix lock on freq counters.

BUG/MAJOR: threads/freq_ctr: fix lock on freq counters.

The wrong bit was set to keep the lock on freq counter update. And the read
functions were re-worked to use volatile.

Moreover, when a freq counter is updated, it is now rotated only if the current
counter is in the past (now.tv_sec > ctr->curr_sec). It is important with
threads because the current time (now) is thread-local. So, rounded to the
second, the time may vary by more or less 1 second. So a freq counter rotated by
one thread may be see 1 second in the future. In this case, it is updated but
not rotated.