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/isc/netmgr/netmgr.c

@@ -216,6 +216,11 @@ isc__nm_in_netthread(void) {
 	return (isc__nm_tid_v >= 0);
 }
 
+void
+isc__nm_force_tid(int tid) {
+	isc__nm_tid_v = tid;
+}
+
 #ifdef WIN32
 static void
 isc__nm_winsock_initialize(void) {