Use random string rather than sha string

Use random string rather than sha string

As discussed on chat
https://chat.civicrm.org/civicrm/pl/r7h6cc7xo78zjgsd8af6ff9tee

this function just generates a hard-to-guess string that is stored
in the civicrm_mailing_event_bounce table. This unique value is
used in conjunction with the unique id value from the table to
determine if it is a match but it's not ever reverse calculated
so we don't need to use a reversable function