cassandra: Add support for ScyllaDB
All features work but snapshot_count_branches, because ScyllaDB does not support user-defined aggregates yet.
Migration tests hang when run after the regular tests, but I can't
figure out why. This should not be an issue for now, as we won't run
Scylla tests on the CI.
Depends on !690 (closed).
Test Plan
It (kind of) works on my machine©
Migrated from D5750 (view on Phabricator)