Indication in the log detail of the source of a result: cache vs a particular upstream vs [etc]

[Cheddoleum]

Had a bit of a time discovering why AdGuard Home kept returning NXDOMAIN for a record which existed, and had existed for some time, on the authoritative server. Turning off "Optimistic Caching" fixed it. it would have been easier to find if the log entry -- in the detail rollover would be fine -- indicted that it was a cached result. More generally, seems like it'd be useful to indicate any source of the result in the log.

[ainar-g]

The initial work is being done in #3772, with some better indication of the system hosts rules in #3846. I'll close this issue, since those two seem to mostly cover the feature requests. Feel free to file new ones if you find anything suspicious!