It would be nice to add an expert API to specify an indexing chain, so that
we can make use of Mike's nice
This patch simply adds a package-protected expert API to IndexWriter and
DocumentsWriter. It adds a inner, abstract class to DocumentsWriter called
IndexingChain, and a default implementation that is the currently used one.
This might not be the final solution, but a nice way to play with different
modules in the indexing chain.
Could you take a look at the patch, Mike?