Comparator that compares based on the Sequence Ids of the
the StoreFiles. Bulk loads that did not request a seq ID
are given a seq id of -1; thus, they are placed before all non-
bulk loads, and bulk loads with sequence Id. Among these files,
the size is used to determine the ordering, then bulkLoadTime.
If there are ties, the path name is used as a tie-breaker.