Do it "more properly": use intrusive list linkage to manage three
indices:
* hash lookup
* recency
* frequency
eviction is frequency based, but in order to avoid things that were
super hot in the past and that are no longer hot clogging up the cache,
eviction will incrementally age out least recently used entries that
haven't been active in the span of some number of get/put operations.
Aging scales down the frequency value to reduce its strength, so an aged
item isn't necessarily immediately a candidate for removal, it just
makes it more likely to be picked up by the frequency based removal as
it goes unused for an extended period.
