metadata-loader: Buffer writes to the storage

https://grafana.softwareheritage.org/goto/Ih3HTVw4z?orgId=1 shows that 15% of its runtime is spent sending (individual) 'raw_extrinsic_metadata_add' requests, which take ~65ms each.

Found while investigating swh/meta#5035, though that's probably not the cause of the main slowness.

This is !52 (closed) but targeted at staging