JGRP-1752: Concurrent message headers modification causes that message is never sent

Under some circumstances the TP protocol may try to add the TP header to message twice concurrently.
This happens for example when the stable message triggers retransmission while the message has been sent right now.
This may result in ArrayOutOfBoundException in Headers._putHeader and/or subsequent NullPointerException in Headers.size(). The retransmission attempt always fails, the message is never delivered. Moreover, keeping this (and possibly following) messages in the transmission table can lead to OOME.