- `:metadata/key-constraints` for those databases which formally track
  PK and FK key constraints. Some databases (eg. Presto/Athena/Starburst
  family) don't support constraints like that, but do support querying
  based on foreign keys (which is the `:foreign-keys` feature).
- `:connection/multiple-databases` for databases where a single
  connection can connect to many databases. This is the case for Athena,
  since it connects to an S3 bucket and exposes all files in the bucket
  as databases through that single connection.
- Also removed a troublesome "all DBs except a huge list" case in the
  SSH tunnel tests with a shorter list of supported drivers.