LogDog: Add Butler stream server package.

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

Index: client/internal/logdog/butler/streamserver/streamserver.go
diff --git a/client/internal/logdog/butler/streamserver/streamserver.go b/client/internal/logdog/butler/streamserver/streamserver.go
new file mode 100644
index 0000000000000000000000000000000000000000..137932f34c8ced6351afa88aff4704ed771a169d
--- /dev/null
+++ b/client/internal/logdog/butler/streamserver/streamserver.go
@@ -0,0 +1,23 @@
+// 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()
+}