This allows a domain to be set in the JetStream server block that sets a domain name.
Once set this signals that any leafnode connections should operate as separate JetStream domains.
Each domain <NAME> is accessible via "$JS.<NAME>.API.>", even when connected to the same domain.
Also for mixed mode you can set a jetstream block now that defines a domain but specifies "enabled: false".
Signed-off-by: Derek Collison <derek@nats.io>