Upgrade SplitHTTP Transport (#3462)

* move to paths instead of querystrings

* permit early data on serverside

* early data for the client, fix context cancellation

transport/internet/splithttp/lazy_reader.go

@@ -0,0 +1,57 @@
+package splithttp
+
+import (
+	"io"
+	"sync"
+)
+
+type LazyReader struct {
+	readerSync   sync.Mutex
+	CreateReader func() (io.ReadCloser, error)
+	reader       io.ReadCloser
+	readerError  error
+}
+
+func (r *LazyReader) getReader() (io.ReadCloser, error) {
+	r.readerSync.Lock()
+	defer r.readerSync.Unlock()
+	if r.reader != nil {
+		return r.reader, nil
+	}
+
+	if r.readerError != nil {
+		return nil, r.readerError
+	}
+
+	reader, err := r.CreateReader()
+	if err != nil {
+		r.readerError = err
+		return nil, err
+	}
+
+	r.reader = reader
+	return reader, nil
+}
+
+func (r *LazyReader) Read(b []byte) (int, error) {
+	reader, err := r.getReader()
+	if err != nil {
+		return 0, err
+	}
+	n, err := reader.Read(b)
+	return n, err
+}
+
+func (r *LazyReader) Close() error {
+	r.readerSync.Lock()
+	defer r.readerSync.Unlock()
+
+	var err error
+	if r.reader != nil {
+		err = r.reader.Close()
+		r.reader = nil
+		r.readerError = newError("closed reader")
+	}
+
+	return err
+}