cassandra: Rewrite content_missing to run queries concurrently.
This is twice as fast, according to https://forge.softwareheritage.org/swh/infra/sysadm-environment#3577
This is the same commit as !736 (closed), rebased on !755 (closed) instead of !727 (closed).
Migrated from D6888 (view on Phabricator)