add XWiki.saveDocuments for saving multiple documents in a single db transaction.

I suggest we add a method
com.xpn.xwiki.XWiki.saveDocuments(List<XWikiDocument>, XWikiContext) throws XWikiException
This method would save a list of documents in a single database transaction.

As I imagine it, setComment() and setMinorEdit() would be called on each document before sending them to saveDocuments.

Since (as far as I can tell) it's impossible to save documents in multiple databases with a single transaction, I think it ought to throw an exception if the user tries. It is draconian but in my opinion the user should be able to rely on it using a single transaction.

I haven't looked in to it very carefully, but it looks like a matter of calling:

store.beginTransaction(context);
try{
    for(XWikiDocument doc : docs){
        orig = doc.getOrigionalDocument();
        store.saveXwikiDoc(doc,context,false);   //bTransaction = false
        (notification stuff)
    }
    store.endTransaction(context, true);
}finally{
    store.endTransaction(context, false);
}

See http://markmail.org/message/7qtx7fnxxgxkpwcl for the started discussion