large number of zones;
allow setting of an environment variable to tune
the number of tasks. default is 8, reccommend
200 zones per task. If you have 200000 zones:
csh: setenv BIND9_ZONE_TASKS_HINT 1000
sh: BIND9_ZONE_TASKS_HINT=1000;
export BIND9_ZONE_TASKS_HINT
Applicable to 9.7, 9.6, auto-tuned in 9.8 and up.