Shorten sent_notifications reply_key

What does this MR do and why?

Use a different format so we can safe some characters in the reply_key. We have switched the partitioned table to use the sliding_list strategy. This means the new table has a partition column whose default value is controlled by the dynamic partition framework. With this strategy we are able to create a unique index for (reply_key, partition) so we can make sure we get no duplicates in single partition.

Until this change is fully rolled out, we are keeping a single partition. So queries that don't specify a partition (like when using SentNotification.record) will still only hit one partition. When the change is rolle out, we will only find records by (reply_key,partition) pairs.

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Related to #514591