core (xmpp): fixed workflow with post_xml_treatments in sendMessage: post_xml_treatments was incorrectly added to workflow using chainDeferred (which only add the callback, so post_xml_treatments own callbacks were not waited). As a result, plugins callback could not be waited as expected before a message was sent. This patch fixes it by not using chainDeferred anymore.

You'll find install instructions in documentation (see doc/installation.rst).

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