Add JSON migrations (CLI migrate cmd) for 0.43

[amaury1093]

Summary

ADR-041 introduces in-place store migrations, but we still maintain JSON dump migrations. Add them to v.043.

Problem Definition

JSON migrations stop at 0.40. There's no migration needed between 0.40-0.42 though. Next one is 0.43.

Proposal

Add JSON migration in the CLI migrate command for v0.43. On the top of my mind, these need to be migrated:

• index supply by denom
• split votes

Note that ADR-028 addresses don't need any JSON migration.

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[aaronc]

Is there any way we can reuse any of the code for in place migrations? Maybe importing old state, migrating in place and exporting? Or is that too complicated?

[amaury1093]

That's probably a bit complicated. The easiest way would be just to extract shared logic into functions. An example: the split votes migration logic is basically this. Put this in a function, and then MigrateStore and MigrateJSON both call it.

[amaury1093]

Turns out index supply by denom doesn't need a JSON migration, since its genesis state didn't change. I added the split votes migration in #8880