backups: root ~~canal~~ path surgery

remove rootPath and getBackupFilePath from the backup target and
make this backend specific.

src/storage/interface.js

@@ -93,6 +93,7 @@ async function listDir(apiConfig, dir, batchSize, marker) {
     assert(typeof marker !== 'undefined');
 
     // Result: array of { fullPath, size }
+    // fullPath is relative to the dir being listed
     throw new BoxError(BoxError.NOT_IMPLEMENTED, 'listDir is not implemented');
 }