Release of probe firmware 5020

• Support for a centos 8 binary repo
• Use the hash of the public key in SOS messages for software probes and anchors, allowing software probes to report SOS messages
• Fix a bug in DNS measurements where a JSON fields got duplicated (measurement busybox 2.2.1)
• Suppress some debug output from dfrm (measurement busybox 2.2.1)
• Make response to a reg. server returning WAIT more robust