core: make large item storage more reliable
core: make large item storage more reliable

When allocating sub-max chunks for the tail end of a large item the
allocator would only look at the exact slab class. If items in a cache
are all exclusively large, these slab classes could be empty. Now as a
fallback it will also check and evict from the largest slab class even
if it doeesn't necessarily want the largest chunk.

memcached/memcached,memcached/memcached,memcached/memcached,memcached/memcached,memcached/memcached