MINOR: ring: make the number of queues configurable

MINOR: ring: make the number of queues configurable

Now the rings have one wait queue per group. This should limit the
contention on systems such as EPYC CPUs where the performance drops
dramatically when using more than one CCX.

Tests were run with different numbers and it was showed that value
6 outperforms all other ones at 12, 24, 48, 64 and 80 threads on an
EPYC, a Xeon and an Ampere CPU. Value 7 sometimes comes close and
anything around these values degrades quickly. The value has been
left tunable in the global section.

This commit only introduces everything needed to set up the queue count
so that it's easier to adjust it in the forthcoming patches, but it was
initially added after the series, making it harder to compare.

It was also shown that trying to group the threads in queues by their
thread groups is counter-productive and that it was more efficient to
do that by applying a modulo on the thread number. As surprising as it
seems, it does have the benefit of well balancing any number of threads.