Fix/rewrite jobs fqn locks

[collado-mike]

Problem

The rewrite_jobs_fqn_table function is inadvertently updating jobs even when no metadata about the job has changed. Under load, this causes significant locking issues, as the jobs_fqn table must be locked for every job update.

Solution

I updated the function to only update the table if the job is a new record or if the symlink_target_uuid is distinct from the previous value. These graphs show the number of blocked transactions and commits over time before and after the change. After applying the change (around 20:10 on these graphs), the number of commits goes way up and the number of active/blocked transactions drops.

I also added a test assertion to validate the update of the jobs_fqn table.

Note: All database schema changes require discussion. Please link the issue for context.

Checklist

• You've signed-off your work
• Your changes are accompanied by tests (if relevant)
• Your change contains a small diff and is self-contained
• You've updated any relevant documentation (if relevant)
• You've updated the CHANGELOG.md with details about your change under the "Unreleased" section (if relevant, depending on the change, this may not be necessary)
• You've versioned your .sql database schema migration according to Flyway's naming convention (if relevant)
• You've included a header in any source code files (if relevant)

[wslulciuc]

Ok, the comments here are much appreciated ❤️

[wslulciuc]

@collado-mike thanks for the fix, and the write up and tests that went along with the change 💯 🥇