LogDog: Add Butler stream server package.

client/internal/logdog/butler/streamserver/streamserver.go

+// Copyright 2015 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 streamserver
+
+import (
+        "io"
+
+        "github.com/luci/luci-go/client/logdog/butlerlib/streamproto"
+)
+
+// StreamServer is an interface to a backgound service that allows external
+// processes to establish Butler streams.
+type StreamServer interface {
+        // Performs initial connection and setup, entering a listening state.
+        Listen() error
+        // Blocks, returning a new Stream when one is available. If the stream server
+        // has closed, this will return nil.
+        Next() (io.ReadCloser, *streamproto.Properties)
+        // Closes the stream server, cleaning up resources.
+        Close()
+}