XWikiDocument#getPreparedXDOM can run the preparation on the same XDOM instance in several threads

Consider the following operations on an instance of XWikiDocument where
xdomCachePrepareDate and xdomCache are initially null.

• Thread 1 calls getXDOM(). this.xdomCache = parseContentNoException(); is executed.
• Thread 1 calls getPreparedXDOM() and starts calling getMacroTransformation().prepare(xdom);
• Thread 2 also calls getPreparedXDOM() and also calls getMacroTransformation().prepare(xdom); as xdomCachePrepareDate is still null

I can think of two things:

• the ideal for performance would probably be to add some read/write locking in `getPreparedXDOM()` to avoid preparing the content several times
• the other possibility is to clone the `this.xdomCache` before preparing it, simpler and `getXDOM()` won't return a half prepared XDOM but cost a bit more