Parallel migration support

[xiaoyao9184]

Since the migration process is divided into start and complete,
the migration process may need weeks or months, or not run complete forever.

Then it is very likely that the previous migration was not completed and the next migration was started.
I have tried do that with examples, but failed.

Failed to start migration: a migration for schema "public" is already in progress

I know this will be complicated, like git branching, but I think it will be very useful.

[HoodieCollin]

I agree that this would be very useful. My team works on multiple features in parallel during a development cycle. With multiple in-progress migrations support, we could have all these features coexist in our staging Env without having to worry about coordinating while developing.