fix: make timeout error implement net.Error (#283)

Author: rgurov

sess.go

@@ -82,10 +82,17 @@ const (
 
 var (
 	errInvalidOperation = errors.New("invalid operation")
-	errTimeout          = errors.New("timeout")
+	errTimeout          = timeoutError{}
 	errNotOwner         = errors.New("not the owner of this connection")
 )
 
+// timeoutError implements net.Error
+type timeoutError struct{}
+
+func (timeoutError) Error() string   { return "timeout" }
+func (timeoutError) Timeout() bool   { return true }
+func (timeoutError) Temporary() bool { return true }
+
 var (
 	// a system-wide packet buffer shared among sending, receiving and FEC
 	// to mitigate high-frequency memory allocation for packets, bytes from xmitBuf