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/BUILD b/drivers/gnmi/BUILD
index efc3f08..f01a9f3 100644
--- a/drivers/gnmi/BUILD
+++ b/drivers/gnmi/BUILD
@@ -7,7 +7,7 @@
     "//protocols/gnmi/stub:onos-protocols-gnmi-stub",
     "//protocols/gnmi/api:onos-protocols-gnmi-api",
     "//protocols/grpc/api:onos-protocols-grpc-api",
-    "//protocols/grpc/proto:onos-protocols-grpc-proto",
+    "//protocols/grpc/utils:onos-protocols-grpc-utils",
 ]
 
 BUNDLES = [