Hashing workaround for unique indexes

[xh3n1]

• create unique indexes on url_hash/regexhash values on new initialisations. Use char type to store hash values.
• make sure to store hash values of URL& Regex fields and URL &regex itself. (apply changes to queries for new entries)
• calculate and store hash for all the rows on upgrade. (before creating indexes)
• create columns for hash values and unique indexes on url_hash/ regexhash values on upgrade.

[xh3n1]

Regarding linktrack table: It doesn't make sense to change the field size and index because it's a legacy table which is not used anymore to add new entries. See uncommented query:

phplist3/public_html/lists/admin/sendemaillib.php

Line 1315 in 0b974ae

/* $req = Sql_Query(sprintf('insert ignore into %s (messageid,userid,forwardid)

and : https://github.com/phpList/phplist3/blob/master/public_html/lists/lt.php#L137-L145