Honour single read per client isc_nm_read() call in the TLSDNS

The TLSDNS transport was not honouring the single read callback for
TLSDNS client.  It would call the read callbacks repeatedly in case the
single TLS read would result in multiple DNS messages in the decoded
buffer.

(cherry picked from commit e3c628d562)

lib/isc/netmgr/tlsdns.c

@@ -1120,6 +1120,10 @@ tls_cycle_input(isc_nmsocket_t *sock) {
 				goto failure;
 			}
 
+			if (atomic_load(&sock->client)) {
+				break;
+			}
+
 			if (pending == 0) {
 				break;
 			}