Drain receive-pack only before verifying a rejection

Reading every push response to EOF made teardown errors fail pushes the report had already confirmed, held back early refusals such as a body limit until the whole pack was sent, and had no size bound. The report is authoritative, so drain only when a concurrent rejection is about to be checked against a fresh target read, cap that drain at maxDrainBytes, and otherwise close as before, ignoring teardown errors.

A failed drain or verification read now leaves the rejection in place with a warning instead of being silent. Link the changelog entries to #123.