IndexWriter today is shared with many classes like BufferedUpdateStream,
DocumentsWriter and DocumentsWriterPerThread. Some of them even acquire locks
on the writer instance or assert that the current thread doesn't hold a lock.
This makes it very difficult to have a manageable threading model.
This change separates out the IndexWriter from those classes and makes them all
independent of IW. IW now implements a new interface for DocumentsWriter to communicate
on failed or successful flushes and tragic events. This allows IW to make it's critical
methods private and execute all lock critical actions on it's private queue that ensures
that the IW lock is not held. Follow-up changes will try to detach more code like
publishing flushed segments to ensure we never call back into IW in an uncontrolled way.