CFS is useful for keeping the open file count down. But, it costs
some added time during indexing to build, and also ties up temporary
disk space, causing eg a large spike on the final merge of an
Since MergePolicy dictates which segments should be CFS, we can
change it to only build CFS for "smallish" merges.
I think we should also set a maxMergeMB by default so that very large
merges aren't done.