Flush all in-memory buffered updates (adds and deletes) to the Directory.

Note: while this will force buffered docs to be pushed into the index, it will not make these docs visible to a reader. Use {@link #Commit()} instead

NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

Flush all in-memory buffered udpates (adds and deletes) to the Directory.