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-int.h

@@ -1111,7 +1111,7 @@ struct isc_nmsocket {
 
 bool
 isc__nm_in_netthread(void);
-/*%
+/*%<
  * Returns 'true' if we're in the network thread.
  */