@VisibleForTesting
  protected ExternalFooterCachesByConf createExternalCaches() {
    return null; // The default ones are created in case of null; tests override this.
  }
Recursively builds 2 maps:
 ID to type description
 child to parent type description ID
@param typeDescription - the considered type description, root on first invocation
@param idMap
@param parentIdMap
ql/src/java/org/apache/hadoop/hive/ql/io/orc/OrcInputFormat.java