Use a ConcatenatedDocumentCollection to combine chunk files

[MikeHopcroft]

We should implement a class that creates a ConcatenatedDocumentCollection from a manifest file listing BitFunnel chunks. This will allow us to build the index in pieces. Orignally, we had though that we would have to build one collection for the entire index.

This new class will be passed to the IndexBuilder command line, the same way we pass ChunkDocumentSequence today.