During Proofreading, do merges “eagerly” without roundtrip

[fm3]

The front-end could maintain its own id mapping parallel to the editable mapping update actions it sends to the back-end. Only when splitting is there a need for a full roundtrip (because the backend has to decide if an edge removal is actually an agglomerate split; and because the min-cut needs an up-to-date agglomerate graph).

[daniel-wer]

Would be fixed by #7654