Add new statistics variable for recursive client high-water
requested to merge 4668-request-to-add-a-new-statistic-recursive-client-high-water-in-rndc-status-and-stats-channel into main
The highwater allows administrators to better tune the recursive clients limit without having to to poll the statistics channel in high rates to get this number.
isc_stats_increment is changed for better tracking.
The effects should be transparent for everyone else.
Closes #4668 (closed)