Please log upstreams on start

It currently logs the fallback, but it would be useful to log the upstreams also to make sure the config is correct:
`2022/01/11 13:59:18 [info] Fallback 0 is https://1.1.1.2:443/dns-query`
Similar lines should be logged for upstreams.

Maybe more useful config options should also be logged, in particular when reading from the yaml config file, i'm not confident all of them are interpreted correctly.