basic tests + lrucrawler -> lru_crawler
return next item from crawler_crawl_q

... instead of return flags and such.
adjust some comments.
don't waste an item flag for crawler

detect via an impossible item: nbytes/nkey are 0. There're other clever ways
we could do this as well (nbytes == maxint, etc).
crawler reclaims stat
control system

nothing internally magically fires it off yet, but now there is an external
command:

lru_crawler crawl [classid]
... will signal the thread to wake up and immediately reap through a
particular class.

need some thought/feedback for internal kickoffs (plugins?)
notes