domap cleanup

## Enhancement

`domap` is the biggest reason why `session` is closely coupled to `domain`. And we even have the following dirty code just to test cases where multiple domain exists.

https://github.com/pingcap/tidb/blob/1770006c2e8f40bcc79068e8d8210de0768a8b83/pkg/session/session.go#L3750-L3758

We can remove it by adding `store.GetOption/SetOption` to add additional context for storage.

Start with the linked section of pkg/session/session.go around lines 3750-3758 and trace how domap, session, domain, and storage are coupled. Review the store interfaces and the proposed GetOption/SetOption context mechanism. Done means the dirty multiple-domain test handling can be removed while storage still receives the required context.

Written by the indexing model from the issue text.