LRU Crawler

LRU crawler.

Background thread that (safely?) crawls stuff on demand.

Needs to be torture tested by mc-crusher before I merge. Just opening a pull-request against myself for shiggles.

EDIT: been torture testing it.. the impact (especially at the default 100 microsecond sleep) is surprisingly low. Isn't crashing while working through millions of items with 15 million gets/sec going on at the same time. Something might still come up during a better prolonged torture test but.. so far it seems okay.