Introduce gopkg.in for go module

This change introduces gopkg.in to allow everyone to use multus
code as library. Currently multus-cni uses version 'v3', hence
it hits go modules limitation, as #504 describes.

This change fix #504 with gopkg.in.

vendor/golang.org/x/net/http2/server.go

@@ -581,13 +581,10 @@ type stream struct {
 	cancelCtx func()
 
 	// owned by serverConn's serve loop:
-	bodyBytes        int64   // body bytes seen so far
-	declBodyBytes    int64   // or -1 if undeclared
-	flow             flow    // limits writing from Handler to client
-	inflow           flow    // what the client is allowed to POST/etc to us
-	parent           *stream // or nil
-	numTrailerValues int64
-	weight           uint8
+	bodyBytes        int64 // body bytes seen so far
+	declBodyBytes    int64 // or -1 if undeclared
+	flow             flow  // limits writing from Handler to client
+	inflow           flow  // what the client is allowed to POST/etc to us
 	state            streamState
 	resetQueued      bool        // RST_STREAM queued for write; set by sc.resetStream
 	gotTrailerHeader bool        // HEADER frame for trailers was seen