Online DDL: new message_timestamp column in schema_migrations table

[shlomi-noach]

Description

This PR introduces schema_migrations. message_timestamp column, which gets updated to current timestamp whenever message gets updated.

The column gets auto-populated with the same query that updates the message. There is thus no actual code change other than an SQL.

Related Issue(s)

Fixes #16632

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.98%. Comparing base (538dd4c) to head (3ca1142).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16633      +/-   ##
==========================================
- Coverage   68.98%   68.98%   -0.01%     
==========================================
  Files        1562     1562              
  Lines      200690   200690              
==========================================
- Hits       138449   138436      -13     
- Misses      62241    62254      +13     

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[harshit-gangal]

why timestamp(6) and not a normal timestamp like other columns?

[shlomi-noach]

Practice tells me that I always end up "upgrading" a timetstamp to timestamp(6). I just appreciate the granularity, especially when it comes to critical points such as cutover where it can really clarify what's going on.