use do_item_remove in lru crawler

the crawler was saving a few operations by directly freeing an item, but lets
use the normal code path instead. The assert looks to ensure item_free() was
actually called as we expected. Though I guess lots of people don't use the
debug binary...
lru_crawler crawl all

... go crawl all available classes.
make stats-cachedump and flush_all ignore crawler
lru_crawler_tocrawl to limit inspected per run

Since we start from the tail, and some slabs could be many millions of items,
it doesn't always make sense to walk the entire thing looking for data to use.

This could be a lot smarter: say specify a percentage, and it'll only crawl a
percentage of used_chunks for that slab for instance.