Implement graceful shutdown of agent.DataAccess (#1008)

* Implements graceful shutdown of agent.DataAccess and underlying Datastore/Logstore/MessageQueue

* adds tests for closing agent.DataAccess and Datastore

api/agent/hybrid/client.go

@@ -248,3 +248,7 @@ func (cl *client) once(ctx context.Context, request, result interface{}, method
 func (cl *client) url(args ...string) string {
 	return cl.base + strings.Join(args, "/")
 }
+
+func (cl *client) Close() error {
+	return nil
+}