Improve the performance of the spam check

Right now the Spam Listener calls XWikiDocument.getXMLContent() and this has 3 unwanted effects:

• It checks and loads a translation from the store which is unnecessary and takes time
• It renders the content of the current document to add it to the XML which is unnecessary and takes time
• It causes a side effect of setting the isMetaDataDirty to true. Seems this is getRenderedContent() which does this, called by toXML() called by getXMLContent().

This third point is causing some other side effects: See the history of http://extensions.xwiki.org/xwiki/bin/view/Extension/MediaWiki/MediaWiki+XML/ which is full of version where the only modification is some description field line ending.