Allow sharing the same gRPC channel between clients

This change introduces a refactoring of the gRPC protocol subsystem that
allows the creation of a gRPC chanel independently of the client, while
allowing multiple clients to share the same channel (e.g. as in Stratum
where we use 3 clients).

Moreover, we refactor the P4RuntimeClient API to support multiple
P4Runtime-internal device ID using the same client. While before the
client was associated to one of such ID.

Finally, we provide an abstract implementation for gRPC-based driver
behaviors, reducing code duplication in P4Runtime, gNMI and gNOI drivers.

Change-Id: I1a46352bbbef1e0d24042f169ae8ba580202944f
diff --git a/drivers/gnmi/src/main/java/org/onosproject/drivers/gnmi/OpenConfigGnmiDeviceDescriptionDiscovery.java b/drivers/gnmi/src/main/java/org/onosproject/drivers/gnmi/OpenConfigGnmiDeviceDescriptionDiscovery.java
index a85cdfb..210a237 100644
--- a/drivers/gnmi/src/main/java/org/onosproject/drivers/gnmi/OpenConfigGnmiDeviceDescriptionDiscovery.java
+++ b/drivers/gnmi/src/main/java/org/onosproject/drivers/gnmi/OpenConfigGnmiDeviceDescriptionDiscovery.java
@@ -23,6 +23,9 @@
 import gnmi.Gnmi.GetRequest;
 import gnmi.Gnmi.GetResponse;
 import org.onlab.packet.ChassisId;
+import org.onosproject.gnmi.api.GnmiClient;
+import org.onosproject.gnmi.api.GnmiController;
+import org.onosproject.grpc.utils.AbstractGrpcHandlerBehaviour;
 import org.onosproject.net.AnnotationKeys;
 import org.onosproject.net.DefaultAnnotations;
 import org.onosproject.net.Device;
@@ -48,7 +51,7 @@
  * supports the gNMI protocol and Openconfig models.
  */
 public class OpenConfigGnmiDeviceDescriptionDiscovery
-        extends AbstractGnmiHandlerBehaviour
+        extends AbstractGrpcHandlerBehaviour<GnmiClient, GnmiController>
         implements DeviceDescriptionDiscovery {
 
     private static final Logger log = LoggerFactory
@@ -58,6 +61,10 @@
 
     private static final String UNKNOWN = "unknown";
 
+    public OpenConfigGnmiDeviceDescriptionDiscovery() {
+        super(GnmiController.class);
+    }
+
     @Override
     public DeviceDescription discoverDeviceDetails() {
         return new DefaultDeviceDescription(