protocol: avoid double buffering

Use buffer only to read the PROXY header.
Users may use they own buffers with they own buffer sizes and pools - connection should respect that.

protocol.go

@@ -44,6 +44,7 @@ type Conn struct {
 	readErr           error
 	conn              net.Conn
 	bufReader         *bufio.Reader
+	reader            io.Reader
 	header            *Header
 	ProxyHeaderPolicy Policy
 	Validate          Validator
@@ -129,9 +130,11 @@ func NewConn(conn net.Conn, opts ...func(*Conn)) *Conn {
 	// For v2 the header length is at most 52 bytes plus the length of the TLVs.
 	// We use 256 bytes to be safe.
 	const bufSize = 256
+	br := bufio.NewReaderSize(conn, bufSize)
 
 	pConn := &Conn{
-		bufReader: bufio.NewReaderSize(conn, bufSize),
+		bufReader: br,
+		reader:    io.MultiReader(br, conn),
 		conn:      conn,
 	}
 
@@ -153,7 +156,7 @@ func (p *Conn) Read(b []byte) (int, error) {
 		return 0, p.readErr
 	}
 
-	return p.bufReader.Read(b)
+	return p.reader.Read(b)
 }
 
 // Write wraps original conn.Write
@@ -335,5 +338,30 @@ func (p *Conn) WriteTo(w io.Writer) (int64, error) {
 	if p.readErr != nil {
 		return 0, p.readErr
 	}
-	return p.bufReader.WriteTo(w)
+
+	b := make([]byte, p.bufReader.Buffered())
+	if _, err := p.bufReader.Read(b); err != nil {
+		return 0, err // this should never as we read buffered data
+	}
+	if _, err := p.bufReader.Discard(len(b)); err != nil {
+		return 0, err // this should never as we read buffered data
+	}
+
+	var n int64
+	{
+		nn, err := w.Write(b)
+		n += int64(nn)
+		if err != nil {
+			return n, err
+		}
+	}
+	{
+		nn, err := io.Copy(w, p.conn)
+		n += nn
+		if err != nil {
+			return n, err
+		}
+	}
+
+	return n, nil
 }