Enhancement

Currently the context usage is a bit of messy. One problem is #56017 and I think a better context usage and cancellation will solve it. After a simple review of current DDL code, I think there should to 2 types of context:

1. the context of a DDL job. All job execution logic should use this context. And another goroutine polling the job state should cancel the context if it's not JobStateRunning.
2. the context of DDL module. Job rollingback, canceling, cleanup, scheduling, execution and other module-level action should use this context. It's canceled when the parent context is canceled which means domain is closing.