Always use the number of CPUS for resolver->ntasks

Since the fctx hash table is now self-resizing, and resolver tasks are
selected to match the thread that created the fetch context, there
shouldn't be any significant advantage to having multiple tasks per CPU;
a single task per thread should be sufficient.

Additionally, the fetch context is always pinned to the calling netmgr
thread to minimize the contention just to coalesced fetches - if two
threads starts the same fetch, it will be pinned to the first one to get
the bucket.

lib/dns/tests/resolver_test.c

@@ -81,7 +81,7 @@ static void
 mkres(dns_resolver_t **resolverp) {
 	isc_result_t result;
 
-	result = dns_resolver_create(view, taskmgr, 1, 1, netmgr, timermgr, 0,
+	result = dns_resolver_create(view, taskmgr, 1, netmgr, timermgr, 0,
 				     dispatchmgr, dispatch, NULL, resolverp);
 	assert_int_equal(result, ISC_R_SUCCESS);
 }