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/grpc/utils/BUILD b/protocols/grpc/utils/BUILD
new file mode 100644
index 0000000..7baaf52
--- /dev/null
+++ b/protocols/grpc/utils/BUILD
@@ -0,0 +1,8 @@
+COMPILE_DEPS = CORE_DEPS + [
+    "//protocols/grpc/api:onos-protocols-grpc-api",
+    "@io_grpc_grpc_java//core",
+]
+
+osgi_jar(
+    deps = COMPILE_DEPS,
+)