Cassandra as a storage backend

Meta task to get my thoughts in order about adding a Cassandra backend to swh-storage. (Started in april)

• have a draft implementation https://forge.softwareheritage.org/source/swh-storage-cassandra/
• benchmark to check the performances are not catastrophic https://forge.softwareheritage.org/source/storage-benchmark-deployment/
• increase test coverage of all behaviors of swh-storage (D 1534 to 1552)
• numeric origin ids
  • define a replacement T1731
  • get rid of numeric origin ids in all storage clients #1816 (closed)
    • non-swh-web clients
    • swh-web
      • queries by origin-id swh-web!182 (closed)
      • paginated queries T1912
  • public API v2 swh-web#1805 (postponed)
• Add the draft Cassandra backend to the docker env
• Run the draft Cassandra backend with production data
• Rewrite the Cassandra backend using the experience learned working on the draft
• Add it to the docker env
• Write a storage proxy component, that queries the two backends (postgres and cassandra) and compares their results, to check they are the same; and run it in the docker env. This will make sure migrating to Cassandra does not introduce regressions
• Run it with production data
• Deploy in production (possibly with the proxy at first)

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Migrated from T1892 (view on Phabricator)