Replace pow check with health check that checks for pow as well
Old pow check was to send an invalid empty attachToTangle command and if pow was enabled IRI returned a http 400. This was replaced by a feature in the GetNodeInfo response. Additionally a bug with 1.7.1 brakes the old pow check. This change is incompatible to old nodes that don't have the GetNodeInfo features. Pow nodes still use the old check as many/all do not support GetNodeInfo.