fixes#10810
The tailnet coordinators don't depend on replicasync, so we can still enable HA coordinators even if the relay URL is unset.
The in-memory, non-HA coordinator probably has lower latency than the PG Coordinator, since we have to query the database, so enterprise customers might want to disable it for single-replica deployments, but this PR default-enables the HA coordinator. We could add support later to disable it if anyone complains. Latency setting up connections matters, but I don't believe the coordinator contributes significantly at this point for reasonable postgres round-trip-time.