Refactor MigrateExecutable::rollback()

Problem/Motivation

While working on #2798363: Implement --idlist options on rollback and messages, I noticed some really wild and crazy things going on in the rollback section of the migrate executable. We loop over the id_map, but then never use the row we are looping over. But as a side "benefit" it advances the rows forward.

Proposed resolution

Let's make the code more extensible and more readable so things like #2798363: Implement --idlist options on rollback and messages can only change a small amount of code, not large hunks of code.

It would be nice, if possible, to backport this to 8.6 as we have a patch (w/ passing tests) in migrate_tools waiting for this feature.

Remaining tasks

• Add a getter for the id_map so we can filter it.
• Refactor the foreach to actually use the value in a loop.

User interface changes

n/a

API changes

n/a

Data model changes

n/a