add revision_metadata logic to storage

• storage.py (as the entry point of storage)
• db.py (as the entry point to db)
• swh-func.sql (the actual sql procedure)
• add the table itself to swh-schema.sql and constraints to swh-indexes.sql
• the data about the indexer tool into swh-data.sql (swh-metadata-detector)
• swh.storage.api.{server,client}
• add json schema into sql/json
• tests evision_metadata
  • add
  • missing
  • get r

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Migrated from T741 (view on Phabricator)