Migration meta issue

[ValuedMammal]

We should document a process for handling database migrations.

Requirements

• Support both up and down migrations. This may require maintaining the old queries.
• Try to batch schema changes into a single migration
• Migrations should happen in a major version release

Known issues

• schema: block table should not allow duplicate heights #8
• schema: anchor block_hash column should have type TEXT #12
• Support wallets with 2+ keychains

[raj-125-beep]

From reading the recent schema issues, I think

For example:

• issue schema: block table should not allow duplicate heights #8 changes the block table primary key to enforce a single entry per height, which would invalidate the existing data layouts.
• issue schema: anchor block_hash column should have type TEXT #12 changes the column type of anchor.block_hash to TEXT, which is semantically correct but still a schema change.

Both seem like cases where applying the fix directly could break existing wallets unless a clear migration strategy is defined. Having documented guidance on when schema changes require migrations, on how to batch them, and when to do them in a major release would help contributors.