LogDog: Add collector service implementation.

server/internal/logdog/collector/coordinator/doc.go

Index: server/internal/logdog/collector/coordinator/doc.go
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Package coordinator implements a minimal interface to the Coordinator service
+// that is sufficient for Collector usage.
+//
+// The interface also serves as an API abstraction boundary between the current
+// Coordinator service definition and the Collector's logic.
+//
+// Cache
+//
+// Coordinator methods are called very heavily during Collector operation. In
+// production, the Coordinator instance should be wrapped in a Cache structure
+// to locally cache Coordinator's known state.
+//
+// The cache is responsible for two things: Firstly, it coalesces multiple
+// pending requests for the same stream state into a single Coordinator request.
+// Secondly, it maintains a cache of completed responses to short-circuit the
+// Coordinator.
+//
+// Stream state is stored internally as a Promise. This Promise is evaluated by
+// querying the Coordinator. This interface is hidden to callers.
+package coordinator