network: use request queue to configure DHCP{4,6} clients

Previously, when UUID is requested for DUID, then the clients are
configured in callback of bus methods.
But now, 'request queue' was implemented, so we can use it to wait until
the product UUID is obtained.

src/network/networkd-queue.h

@@ -28,6 +28,8 @@ typedef enum RequestType {
         REQUEST_TYPE_BRIDGE_MDB,
         REQUEST_TYPE_CREATE_STACKED_NETDEV,
         REQUEST_TYPE_DHCP_SERVER,
+        REQUEST_TYPE_DHCP4_CLIENT,
+        REQUEST_TYPE_DHCP6_CLIENT,
         REQUEST_TYPE_IPV6_PROXY_NDP,
         REQUEST_TYPE_NEIGHBOR,
         REQUEST_TYPE_NEXTHOP,