feat: add in-memory cache for DNS #6073
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This adds "stale-while-revalidate" in-memory cache for DNS. Instead of calling
tokio::net::lookup_hostwe use previous result oftokio::net::lookup_hostimmediately and spawn revalidation task in the background. This way all lookups after the first successful return immediately.Most of the time results returned by resolvers are the same anyway, but with this cache we avoid waiting 60 second timeout if DNS request is lost. Common reason result may be different is round-robin DNS load balancing and switching from IPv4 to IPv6 network. For round-robin DNS we don't break load balancing but simply use a different result, and for IPv6 we anyway likely have a result in persistent cache and can use IPv4 otherwise.
Especially frequest should be the case when you send a message over SMTP and SMTP connection is stale (older than 60 s), so we open a new one. With this change new connection will be set up faster as you don't need to wait for DNS resolution, so message will be sent faster.