MINOR: listener: pre-compute some thread counts per bind_conf

MINOR: listener: pre-compute some thread counts per bind_conf

In order to quickly pick a thread ID when accepting a connection, we'll
need to know certain pre-computed values derived from the thread mask,
which are counts of bits per position multiples of 1, 2, 4, 8, 16 and
32. In practice it is sufficient to compute only the 4 first ones and
store them in the bind_conf. We update the count every time the
bind_thread value is adjusted.

The fields in the bind_conf struct have been moved around a little bit
to make it easier to group all thread bit values into the same cache
line.

The function used to return a thread number is bind_map_thread_id(),
and it maps a number between 0 and 31/63 to a thread ID between 0 and
31/63, starting from the left.