moodle-tool_mergeusers - two improvements

    * This tool now supports keys of more than 2 terms on the key/index. To do so:
       * the config field 'otherfield' (string) becomes 'otherfields' (array(string1, string2,...)).
       * when field 'both' appears indicates a field on 'otherfields' may appear also as user-related field.
    * A new compound index is included on the table role_assigments (compound index formed by userid, context and role).