Deprecating support of OpenVAS network mode by December 31st 2020

The experimental feature of the network scan mode of OpenVAS was developed to force scanning steps into parallel processing, especially for wrapped external tools like Nmap. The concept did not work out as expected but it helped to find advanced concepts. Wrapping tools in VTs did not deliver the desired seamless integration into the scan process. Greenbone rather started into micro-service architecture to bring bottlenecks into parallel seamless integration. Now it is time to finally drop the network scan mode. Version 20.08 will be the first one without. On the feed side, the use of the functionality will be removed by December 31st 2020 for all older releases.