Endpoint to detect whether Fuseki is in sync or not

[tomkxy]

Although issue #171 is resolved (many thanks for that), we still would need an endpoint through which we would be able to detect whether a Fuseki is syncing or not.

This would allow to exclude a Fuseki instance from a load balancer while it is syncing, e.g. in a Kubernetes cluster this could be used to implement a readiness probe.