LogDog: Add Butler stream server package.

client/logdog/butlerlib/streamproto/magic.go

Index: client/logdog/butlerlib/streamproto/magic.go
diff --git a/client/logdog/butlerlib/streamproto/magic.go b/client/logdog/butlerlib/streamproto/magic.go
index 4eb73615c90b80693aa3c7dfd0572e6d67f6017a..7930084575345d516b6b9a137faab6339f78ec29 100644
--- a/client/logdog/butlerlib/streamproto/magic.go
+++ b/client/logdog/butlerlib/streamproto/magic.go
@@ -7,5 +7,11 @@ package streamproto
 var (
 	// ProtocolFrameHeaderMagic is the number at the beginning of streams that
 	// identifies the stream handshake version.
+	//
+	// This serves two purposes:
+	//   - To disambiguate a Butler stream from some happenstance string of bytes
+	//     (which probably won't start with these characters).
+	//   - To allow an upgrade to the wire format, if one is ever needed. e.g.,
+	//     a switch to something other than recordio/JSON.
 	ProtocolFrameHeaderMagic = []byte("BTLR1\x1E")
 )