Add a task queue task router to reduce amount of boilerplate.

scheduler/appengine/engine/internal/tq_tasks.proto

Index: scheduler/appengine/engine/internal/tq_tasks.proto
diff --git a/scheduler/appengine/engine/internal/tq_tasks.proto b/scheduler/appengine/engine/internal/tq_tasks.proto
new file mode 100644
index 0000000000000000000000000000000000000000..b90ca76863155f3c2b6732f6d5b01c379938c516
--- /dev/null
+++ b/scheduler/appengine/engine/internal/tq_tasks.proto
@@ -0,0 +1,29 @@
+// Copyright 2017 The LUCI Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+
+package internal.tq_tasks;
+
+// TODO(vadimsh): These will change.
+
+message TickLaterTask {
+  string job_id = 1;
+  int64 tick_nonce = 2;
+}
+
+
+message StartInvocationTask {
+  string job_id = 1;
+}