Add low contention stats counter

Add low contention stats counter

In the current statistics counter implementation, the statistics are
backed by an array of counters, which are updated via atomic operations.
This leads to contention, especially on high core count
machines.

This commit introduces a new isc_statsmulti_t counter that keeps a
separate array per thread. These counters are then aggregated only when
statistics are queried, shifting work off the critical path.

These changes lead to a ~2% improvement in perflab.