swh.indexer.recompute: Add storage primary_key option for corruption check

This no longer fetches metadata from contents. The contents passed as
parameter should be self-contained and in adequation with the
'primary_key' configuration option. As of now, that primary key is
'sha1'.

Unknown or corrupted contents are skipped.

Add some other improvments:

• Improve docstrings
• Align option names with _ as per usual conventions
• Add a batch_size to retrieve contents' blobs
• Add another batch_size for the update contents operations

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