Rework DNS resolving

[Snawoot]

Purpose of proposed changes

• Fix inconsistencies regarding DNS cache: now (potential) resolves of both requested hosts and upstream proxies are done the same way, using same DNS cache.
• Bump default DNS cache time to 10 seconds, effectively enabling it by default. It should reduce strain on upstream forwarders such as home gateways which may refuse repeated queries for the same name.
• Allow resolving of upstream proxy name also be private.
• Simplify code and logic.

Essential steps taken

• Introduce forced DNS resolving before and after chain of proxy and filter dialers.
• Unify DNS resolution caching in the same caching resolver rather than having it inside separate caching resolving dialer.