use LRU for packet cache instead of persistent cache

use LRU for packet cache instead of persistent cache

The primary motivation is that packet cache is shared with records,
but it isn't useful for improving performance of popular domain lookups.

Negative answers (with DNSSEC) are typically around 1kB, so it doesn't
take many queries to force a full cache flush, including useful record data.

This changes the packet cache to use a fixed-size LRU, which is more resilient
to random prefix attacks, but it isn't persistent or shared between processes.