strict_dns: allow to keep stale targets for a specified amount of time

Problem: When a DNS does not return all targets on each resolve, strict_dns will continually be adding and draining targets. For this case logical_dns is better, but it's still not great, it keeps track of a single logical target even though connections are made to several, for the given resolution interval all connections and traffic will flow to a single IP address, also I really don't know how outlier_detection and other advanced features can play with a logical_dns cluster. original_dst cluster supports cleaning unused targets and be an option, but it's also probably unsafe since it allows connecting to any IP address.

Proposed Solutions:

• Adding support for cleanup_interval for strict_dns clusters like original_dst does, and remove them when they have not been returned for a given time.
• Adding a "mixed dns" cluster which supports this sort of behavior.