Add a strict Citus mode

It could be useful if we had a strict Citus mode that enforced properties that enable us to always sync metadata and move shards.

- Citus tables must have replica identity
- Disallow objects that Citus does not replicate, except when created by an extension
- New tables immediately become Citus local tables
- Functions are immediately replicated to workers

This could enable a number of features:
- Automatic background re-balancing
- Zero-downtime upgrades / migrations -- coordinator can be fully migrated through metadata syncing and shard moves
- Metadata syncing by default