direct reclaim mode for evictions

Only way to do eviction case fast enough is to inline it, sadly.
This finally deletes the old item_alloc code now that I'm not intending on
reusing it.

Also removes the condition wakeup for the background thread. Instead runs on a
timer, and meters its aggressiveness by how much shuffling is going on.

Also fixes a segfault in lru_pull_tail(), was unlinking `it` instead of
`search`.