Make sure we have a migration path off CockroachDB

[leoluk]

Relying on what is now a proprietary database is most certainly not going to end well. There's nothing inherently wrong with the new Cockroach Labs licensing structure, but given our project's goals, it seems ill-advised to become locked into it.

We should evaluate alternatives and make sure (via appropriate tests) that we don't use Cockroach-specific features so we have a migration path to another Postgres-compatible HA solution.

Potential alternatives:

• Staying on the BSL CockroachDB version, which will eventually become Apache2. Oxide has forked it.
• Another flavor of consensus-based OLTP DB.
  • TiKB/TiDB.
    • MySQL-flavored.
    • No fully serializable transactions (requires manual locking via SELECT FOR UPDATE or a CAS).
  • YugabyteDB.
  • Neon.
  • ...
• Single-writer Postgres with synchronous replication and our own etcd-based leader election.