Free GRPC resources on stub close (#69)

Lastest versions of GRPC does not include the shutdown and destroy channel
on _del_ call. (grpc/grpc#12531).

Author: jimyx17

pydgraph/client_stub.py

@@ -70,5 +70,9 @@ def check_version(self, check, timeout=None, metadata=None,
 
     def close(self):
         """Deletes channel and stub."""
+        try:
+            self.channel.close()
+        except:
+            pass
         del self.channel
         del self.stub