metadata-loader: Buffer writes to the storage

Closed vlorentz requested to merge vlorentz/swh-charts:metadata-loader-buffer-staging into staging\?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

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