Migration meta issue

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

- [x] #8
- [x] #12
- [x] #19
- [ ] Support wallets with 2+ keychains

未指定任何文件、测试或入口点。首先检查现有的迁移实现以及此 issue 中已勾选和未勾选的项目；完成意味着记录一个涵盖向上和向下迁移、批量处理 schema 变更以及 major 版本发布的流程。

由索引模型根据 Issue 内容生成。