Create per-thread task and memory context for zonemgr

Previously, the zonemgr created 1 task per 100 zones and 1 memory
context per 1000 zones (with minimum 10 tasks and 2 memory contexts) to
reduce the contention between threads.

Instead of reducing the contention by having many resources, create a
per-nm_thread memory context, loadtask and zonetask and spread the zones
between just per-thread resources.

Note: this commit alone does decrease performance when loading the zone
by couple seconds (in case of 1M zone) and thus there's more work in
this whole MR fixing the performance.

lib/ns/update.c

@@ -19,7 +19,6 @@
 #include <isc/serial.h>
 #include <isc/stats.h>
 #include <isc/string.h>
-#include <isc/taskpool.h>
 #include <isc/util.h>
 
 #include <dns/db.h>