Implement a generic infrastructure called DocumentChangeSet, that allows scheduling changes across a whole set of different documents, no matter whether they are currently open or closed.

It eliminates duplicates, and detects conflicts, with graceful recovery on problems.
When a change problem/inconsistency appears, all changes on all scheduled files are reverted.
This does not use smart-ranges yet, so no editing is allowed during a run.

This will be used for selective refactoring that touches many different files. For example simple declaration-renaming.