update the flow control windows of streams opened in 0-RTT

The server might have increased the initial flow control window. We need
to make sure to inform all streams opened during during the 0-RTT
period.

streams_map_generic_helper.go

@@ -11,6 +11,7 @@ import (
 // This definition must be in a file that Genny doesn't process.
 type item interface {
 	generic.Type
+	updateSendWindow(protocol.ByteCount)
 	closeForShutdown(error)
 }