Recreate p_sent_notifications.reply_key index

Mario Celirequested to merge
584967-recreate-reply_key-index into master

What does this MR do and why?

A unique index on (reply_key, partition) was dropped on the table in order to satisfy a cells requirement on unique indexes with !214140 (merged). But we still need an index on the reply_key column as we search on the table by reply_key

Related to #584967 (closed)

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