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 (closed), though that's probably not the cause of the main slowness.

Please don't deploy this before swh/devel/swh-loader-metadata!16 (merged) or it would make the metrics "miss" the actual time spent writing