improve cache flushing/eviction mechanism

Right now cache flushing is kind of dumb because it flushes whole cache as soon as it is full. This does not behave very well under high load because it leads to bursts of queries.

We need to find a way to implement proper cache eviction so items can be removed incrementally instead of dropping whole cache.

See https://gitlab.labs.nic.cz/knot/knot-resolver/wikis/Knot-Resolver-Cache-Garbage-Collector