Multi data path config can cause a shard to be perceived as corrupted

Multi data path config support writes a file to a data location based on the available size (by default). There is a Lucene file called segments.gen that has the same name, and only in that case, we need to make sure we alway write it to the same data location, otherwise, the index will have multiple segments.gen files, and the shard can seem to be corrupted.

The message if this case happens is that segments_xxx file was not found, in which case, a find for segments.gen can yield multiple files. Deleting the segments.gen files will cause the shard to recover properly (as its an extra protection layer to resolve the segments header by Lucene)

Make sure the segments.gen file is writtne to the same directory every time
fixes #4674