Gateway state sync mechanism

[jvstme]

Introduce a way to reconcile gateway replica state based on the current list of services. This allows newly started or newly recovered gateway replicas to learn about existing services. It also improves fault tolerance, since an unavailable gateway replica no longer blocks the service lifecycle - services and service replicas can still be registered on the remaining healthy gateway replicas.

Behavioral details:

• Run submission no longer waits for service registration before the run starts provisioning. This has both advantages (provisioning and service registration can happen in parallel) and disadvantages (compute may be provisioned for a service that will be terminated a moment later due to a registration error). This behavior may be revised in the future.
• A service or a service replica is terminated with the gateway_error termination reason if it failed to be registered on all gateway replicas.
• If registration succeeded at least on one gateway replica, registration on the remaining gateway replicas will be retried indefinitely.
• Rolling deployments only proceed with terminating a service replica once the replacement replica is successfully registered on all gateway replicas.
• Registration and unregistration tracking, including per-gateway-replica registration error messages, is available in events.

#3959