Reduce the number of clientmgr objects created

Previously, as a way of reducing the contention between threads a
clientmgr object would be created for each interface/IP address.

We tasks being more strictly bound to netmgr workers, this is no longer
needed and we can just create clientmgr object per worker queue (ncpus).

Each clientmgr object than would have a single task and single memory
context.

lib/ns/tests/query_test.c

@@ -46,6 +46,8 @@ _setup(void **state) {
 
 	UNUSED(state);
 
+	isc__nm_force_tid(0);
+
 	result = ns_test_begin(NULL, true);
 	assert_int_equal(result, ISC_R_SUCCESS);
 
@@ -56,6 +58,8 @@ static int
 _teardown(void **state) {
 	UNUSED(state);
 
+	isc__nm_force_tid(-1);
+
 	ns_test_end();
 
 	return (0);