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.

This is !52 (closed) but targeted at staging

added 1 commit

• 1d6fdcc0 - metadata-loader: Buffer writes to the storage

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@vlorentz, i'll do the merging (in branches) and deployment [1] if you don't mind [synchronizing with the deployment of your other mr too ;)

[1] swh/infra/sysadm-environment#4887 (closed)

mentioned in issue swh/infra/sysadm-environment#4887 (closed)

cherry-picked @ ac5f3d27

closed