The recursor cache needs to check stale when updating the ns type cache.

[zjs604381586]

Short description

When an NS record already exists in the recursor cache and the NS record is replaced again, the TTD setting of the NS record should check the stale flag of CacheEntry. Otherwise, the TTL of the NS cache will always be s_serveStaleExtensionPeriod(30s). Then the new NS cache will expires quickly, causing delays in iterative DNS resolution

Checklist

I have:

• [x] read the CONTRIBUTING.md document
• [x] compiled this code
• [x] tested this code
• [ ] included documentation (including possible behaviour changes)
• [ ] documented the code
• [ ] added or modified regression test(s)
• [ ] added or modified unit test(s)
• [ ] checked that this code was merged to master