cache: clip ttl to hour..ttl..week

[hannesm]

this is likely controversial -- reasoning behind this is https://00f.net/2019/11/03/stop-using-low-dns-ttls/ and the observation that e.g. github.com has ridiciously low ttls for no good reason... this PR is mostly a reminder that the current behaviour is not optimal for most setups, and should be refined. an option could be to provide configuration arguments (though dns cache doesn't have configuration atm); or we decide to merge and use the increased lower ttl. i tried this at the reproducible builds summit in december 2019 in marrakesh and didn't hear any complaints -- i suspect nobody noticed...

[cfcs]

I think it's controversial, it will make all users of the cache (e.g. the client?) useless for a lot of use-cases, for instance when taking part in a Consul network (where low TTL is desired) etc.

Seems to me that this should be something configurable in a resolver (and maybe also client?) rather than hardcoded.

[hannesm]

I agree this should be somehow configurable, and in some scenarios it is a good idea, while it is not such a good idea in other scenarios. let's keep this PR open for now (it is as well not urgent to get the clip in - but otherwise we'll forget about it).