stop using slab class 255 for page mover
stop using slab class 255 for page mover

class 255 is now a legitimate class, used by the NOEXP LRU when the
expirezero_does_not_evict flag is enabled. Instead, we now force a single bit
ITEM_SLABBED when a chunk is returned to the slabber, and
ITEM_SLABBED|ITEM_FETCHED means it's been cleared for a page move.

item_alloc overwrites the chunk's flags on set. The only weirdness was
slab_free |='ing in the ITEM_SLABBED bit. I tracked that down to a commit in
2003 titled "more debugging" and can't come up with a good enough excuse for
preserving an item's flags when it's been returned to the free memory pool. So
now we overload the flag meaning.

samanbarghi/memcached,guikcd/memcached,gencer/memcached,gencer/memcached,samanbarghi/memcached,gencer/memcached,memcached/memcached,memcached/memcached,gencer/memcached,elfchief/memcached,bpzhang/memcached,dormando/memcached,dormando/memcached,rmccullagh/memcached,bpzhang/memcached,rmccullagh/memcached,elfchief/memcached,guikcd/memcached,guikcd/memcached,elfchief/memcached,memcached/memcached,memcached/memcached,elfchief/memcached,rmccullagh/memcached,elfchief/memcached,dormando/memcached,guikcd/memcached,dormando/memcached,dormando/memcached,memcached/memcached,rmccullagh/memcached,gencer/memcached,bpzhang/memcached,guikcd/memcached,rmccullagh/memcached,samanbarghi/memcached,bpzhang/memcached,samanbarghi/memcached