Use StoreFileTracker when splitting and merging

Changes MergeTableRegionProcedure and SplitTableRegionProcedure to use the underlying StoreFileTracker implementation to select the valid files on regions to be merged/split, rather than direct listing those from file system. It also changes merge/split commit logic in HRegionFileSystem, to add the links/refs on resulting split/merged regions to the StoreFileTracker.