It buffers 'raw_extrinsic_metadata' objects to reduce write latency.

It's currently pulled from the rsvndump image but it should be explicitely defined. It's
already the case for the Dockerfile of the save-code-now apps for example.

Refs swh/infra/sysadm-environment#4894

Ref. swh/infra/sysadm-environment#4888

Refs. swh/infra/sysadm-environment#4890

Refs. swh/infra/sysadm-environment#4886

Refs. swh/infra/sysadm-environment#4886

Refs. swh/infra/sysadm-environment#4886

Refs. swh/infra/sysadm-environment#4886

Refs. swh/infra/sysadm-environment#4886

Refs. swh/infra/sysadm-environment#4886