Refactor StoreFileScanner creation to builder pattern

// which args are the most relevant to this?
-      List<StoreFileScanner> sfScanners = StoreFileScanner.getScannersForStoreFiles(sfs,
-        cacheMobBlocks, true, false, false, readPt);

// ah, very clearly we are using defaults except for the caching settings
+      List<StoreFileScanner> sfScanners = new StoreFileScanner.ListBuilder(sfs, readPt)
+          .withCacheBlocks(cacheMobBlocks).build();