Add dedupe() method for a field mapping. This assures that the value is unique, even if source data contains duplicates. Commonly used for usernames. Uniqueness is currently achieved by appending a number (e.g. example1). In the future we can expand to custom replacement patterns such as email address: sally1@example.com. Dedupe can be used on any mapping.

Record correct uid in node_revisions table. We rarely want the currently logged in user, which is what Drupal assumes.

Smarten --all so it can take a value indicating which Migration to start from. Then the remaining on the list are processed.