add mirror health check

[lanefu]

add async health check to active mirrors, remove active mirror if heatlcheck fails

[lanefu]

not sure if just check availability or check for full sync

https://pastebin.com/XE0LB9uX

[igorpecovnik]

Improved version
https://pastebin.com/b4jwzRfb

[lanefu]

integrate with this somehow?