replay: do copy invalid objects when they exists in the archive

If an object is detected as invalid (hash mismatch), then instead of simply dropping it (and log the issue in the error reporter), optionally check the existence of the object in the archive; if it does indeed exists there, do perform the replication.

The check uses the public API; to enable this feature, the replayer configuration section must define the API url, e.g. api_url: https://archive.softwareheritage.org/api/1