[keyserver] Clean up fetchMessageInfos using ROW_NUMBER window function

Summary:
From reviewing some StackOverflow answers, I learned that MariaDB 10.8 supports ROW_NUMBER, which wasn't previously supported in MySQL 5.7. We can simplify this query while maintaining the same performance.

Depends on D4674

Test Plan: I first tested this stack locally in my environment to make sure the queries worked. Then I patched it onto my production keyserver and confirmed performance was improved

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: adrian, abosh

Differential Revision: https://phab.comm.dev/D4675