needs_update in the migrate_map_% tables is named/used inconsistently

The 'needs_update' field in the 'migrate_map_%' tables is named incorrectly and being used for competing data.

Code from includes/map.inc:

/**
 * Codes reflecting the current status of a map row.
 */
const STATUS_IMPORTED = 0;
const STATUS_NEEDS_UPDATE = 1;
const STATUS_IGNORED = 2;
const STATUS_FAILED = 3;

Three statuses answer the question "What happened to this row during the most recent update?" One answers "Should this row be updated no matter what the next time the migration is run?" What happens when you need to know which rows failed after you set the row to be updated again? You can't.

The (a?) solution to this issue is to create an 'import_status' row and use the original 'needs_update' field as a flag (as it seemed to be named/designed for.) Thoughts?

Attached patch separates needs_update flag data from row import status, includes hook_update_7208() to adjust existing installs. The syntax of the MigrateMap::saveIDMapping() function has been changed to include separate import_stats and needs_update values. Variable defaults now come from the database defaults rather than function and database defaults to better match Drupal standards: If a passed value is NULL, it will not be updated in the database. No more losing your $hash value to set a $rollback_action.

I've checked the following contrib modules: commerce_migrate, g2migrate, migrate_d2d, migrate_extras, phpbb2drupal, TYPO3_migrate, and wordpress_migrate. Only the current dev of migrate_d2d uses the saveIDMapping() function, so a small patch is required for it.

The above code from includes/map.inc changes to:

  /**
   * Map row last import status: Import sucessful.
   */
  const STATUS_IMPORTED = 0;
  /**
   * DEPRECATED Map row last import status: Needs Update.
   * @todo Remove this value in next version: 2.7 or 3.0.
   */
  const STATUS_NEEDS_UPDATE = 1;
  /**
   * Map row last import status: Import sucessful. Ignored during last import.
   */
  const STATUS_IGNORED = 2;
  /**
   * Map row last import status: Failed during last import.
   */
  const STATUS_FAILED = 3;
  /**
   * Map row last import status: Not imported. The row default.
   */
  const STATUS_NOT_IMPORTED = 4;

  /**
   * Map row needs update: False, update not needed.
   */
  const NEEDS_UPDATE_FALSE = 0;
  /**
   * Map row needs update: True, update needed.
   */
  const NEEDS_UPDATE_TRUE = 1;