pt-table-sync is not considering unsorted enum fields in indexes for calculating chunk boundaries

As concluded in comments of PT-1591 what has been added for pt-online-schema-change and pt-checksum-table (CONCAT when unsorted enum field is part of the index to have proper ordering for chunk boundaries) is not available in pt-table-sync because it uses some other methods.

Relevant comments are:

https://jira.percona.com/browse/PT-1591?focusedCommentId=228116&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-228116

and

https://jira.percona.com/browse/PT-1591?focusedCommentId=228123&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-228123