swh.indexer.rehash: Respect semantics regarding hashes (re)computation

For all contents:

• 'compute_checksums' option define checksums to (re)compute
• check the content's already defined checksums against that list
• If some are in 'compute_checksums' and already defined for that content, recompute those only if recompute_checksums is true.
• Compute all other checksums mentioned in 'compute_checksums' for that content
• update the content's metadata in db

Related T692