Fix discovery managers to be properly cancelled

[zzJinux]

for range m.ctx.Done() won't let the program touch its body; m.ctx.Done() will only close.

Although I don't know what m.cancelDiscoverers exaclty does, it seems to me that it must be called for graceful shutdown.

Here is the simple demo for comparing: https://go.dev/play/p/UDzf26J0zcs

@roidelapluie

[zzJinux]

I don't have idea why three of those checks fail. It seems temporary network connection issue I guess?

[roidelapluie]

Thanks, I will test this later

[roidelapluie]

Thanks!