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/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java b/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java
index b0e0071..4794570 100644
--- a/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java
+++ b/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java
@@ -25,6 +25,6 @@
  */
 @Beta
 public interface GnmiController
-        extends GrpcClientController<GnmiClientKey, GnmiClient>,
+        extends GrpcClientController<GnmiClient>,
         ListenerService<GnmiEvent, GnmiEventListener> {
 }