Make DNSProxy RFC 8767 compliant - Optimistic caching/stale records

[iJorgen]

Background:

Optimistic caching and serving stale records are great for performance, but in today's implementation it's depending on the cache to be filled up before anything is deleted. If using a big cache-size a record could live in the cache for weeks leading to old data.

Suggestion for three configurable parameters to make DNSProxy RFC 8767 compliant and give the DNS-admin full control:

1. "stale-answer-ttl"
Defines the TTL for a stale record. Today it's hardcoded to 10 seconds, but RFC 8767 recommends a value of 30 seconds for stale records.

2. "max-stale-ttl"
Defines how long a stale record can live in the cache. Recommended values from RFC 8767 is 1-3 days.

3. "stale-refresh-time"
The period for serving a stale answer while updating it upstream. RFC 8767 recommends 30 seconds or higher.

Sources:

https://www.rfc-editor.org/rfc/rfc8767
https://www.isc.org/blogs/2020-serve-stale/