CachingResolver: stop retrying permanently failing hosts

[guizmaii]

Problem

When expireAction == Refresh, a host that fails DNS resolution is retried every unknownHostTtl (default 1 minute) indefinitely. Each retry consumes a semaphore permit and makes a blocking system DNS call. For hosts that will never resolve, this wastes resources forever.

Proposal

Add a max retry count to CachingResolver. After a configurable number of consecutive failed refreshes, the entry would be dropped from the cache instead of being retried.

This would require:

• A new failureCount field on CacheEntry, incremented in refreshOrDropEntries each time a failed entry is refreshed
• A new maxRetries config field in DnsResolver.Config (with a sensible default)
• When failureCount >= maxRetries, drop the entry regardless of expireAction

Alternatives

• Max failure TTL: Track when the host first started failing and drop after a duration ceiling. Similar complexity (new field on CacheEntry) but uses time rather than count.
• Exponential backoff: Progressively increase the retry interval. Reduces resource usage without a hard cutoff, but adds complexity.

@987Nabil Which solution do you prefer?