FeedReader UpdateThread throws NPE in 1.8.1+

The FeedReader clones and passes the XWikiContext to the updating thread, which uses XWiki.getDocument("fullname", context) to get the feed aggregator and feed entry classes.
getDocument() doesn't use the passed context but does the resolution of the fullName using a DocumentNameFactory, which uses the ExecutionContext.getProperty("xwikicontext"), which is null now, since it's a different thread.

One solution, to prevent the DocumentNameFactory to use the "xwikicontext" property, would be to make sure to pass the full classname everytime a getDocument would be used. However, it might not be a failsafe solution, since other things might be needed from the ExecutionContext, in other code.

The feedreader plugin needs to be rewritten according to the new core architecture. And, of course, pulled out of the core and implemented as a real plugin.