Storage database migration tooling

Rationale

We currently use untested handwritten and maybe-run database migration scripts for the main storage database.

Being already a cumbersome solution for the current production setup, it will pupate a real nightmare once we (at last) have proper mirrors here and there.

We need to use better tooling for this, and define a proper migration strategy.

Note that this is also linked to //The Journal Migration Problem//, aka swh-journal#1279 (closed)

Objectives

How to:

• test the migrations
• run the migrations
• avoid mismatches between python and sql
• properly handle our different flavors of database (write db vs. read replica)

Tooling

Investigate what tooling can be helpful for this.

• https://sqitch.org/
• ...

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Migrated from T2524 (view on Phabricator)