core: Add slab allocator
core: Add slab allocator

* Slab allocator resembles the one used by stock memcached, where
it's composed of slab classes and slab classes are composed of
chunks of the same size.
* Per-slab-class LRU is also available.
* Slab allocator exports stats to collectd.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@cloudius-systems.com>

joerg84/seastar,kangkot/scylla,senseb/scylla,raphaelsc/seastar,tempbottle/seastar,shyamalschandra/seastar,acbellini/scylla,scylladb/scylla,dwdm/seastar,raphaelsc/scylla,linearregression/scylla,anzihenry/seastar,jonathanleang/seastar,mixja/seastar,duarten/scylla,stamhe/scylla,shaunstanislaus/scylla,anzihenry/seastar,kangkot/scylla,asias/scylla,leejir/seastar,tempbottle/seastar,flashbuckets/seastar,victorbriz/scylla,printedheart/seastar,duarten/scylla,aruanruan/scylla,norcimo5/seastar,joerg84/seastar,xtao/seastar,dreamsxin/seastar,joerg84/seastar,acbellini/seastar,stamhe/scylla,dwdm/scylla,aruanruan/scylla,bowlofstew/scylla,cloudius-systems/seastar,acbellini/scylla,slivne/seastar,justintung/scylla,tempbottle/scylla,ducthangho/imdb,linearregression/seastar,hongliangzhao/seastar,rluta/scylla,hongliangzhao/seastar,linearregression/scylla,scylladb/scylla,flashbuckets/seastar,tempbottle/scylla,ducthangho/imdb,asias/scylla,gwicke/scylla,kjniemi/scylla,koolhazz/seastar,tempbottle/seastar,avikivity/seastar,kjniemi/seastar,xtao/seastar,bowlofstew/seastar,shyamalschandra/seastar,flashbuckets/seastar,koolhazz/seastar,jonathanleang/seastar,shyamalschandra/seastar,chunshengster/seastar,phonkee/scylla,scylladb/seastar,eklitzke/scylla,respu/scylla,victorbriz/scylla,xtao/seastar,bowlofstew/seastar,acbellini/scylla,rluta/scylla,anzihenry/seastar,rentongzhang/scylla,glommer/scylla,avikivity/seastar,scylladb/seastar,mixja/seastar,kjniemi/seastar,bowlofstew/scylla,stamhe/seastar,avikivity/scylla,scylladb/scylla,eklitzke/scylla,asias/scylla,wildinto/scylla,mixja/seastar,stamhe/scylla,wildinto/scylla,chunshengster/seastar,bowlofstew/scylla,eklitzke/scylla,linearregression/seastar,bzero/seastar,acbellini/seastar,guiquanz/scylla,raphaelsc/scylla,ducthangho/imdb,acbellini/seastar,kangkot/scylla,wildinto/scylla,aruanruan/scylla,senseb/scylla,scylladb/scylla-seastar,gwicke/scylla,kjniemi/seastar,stamhe/seastar,dreamsxin/seastar,norcimo5/seastar,avikivity/seastar,leejir/seastar,justintung/scylla,tempbottle/scylla,stamhe/seastar,scylladb/scylla,slivne/seastar,raphaelsc/seastar,wildinto/seastar,koolhazz/seastar,printedheart/seastar,wildinto/seastar,linearregression/seastar,shaunstanislaus/scylla,cloudius-systems/seastar,syuu1228/seastar,justintung/scylla,sjperkins/seastar,rluta/scylla,shaunstanislaus/scylla,respu/scylla,glommer/scylla,rentongzhang/scylla,scylladb/seastar,linearregression/scylla,senseb/scylla,scylladb/scylla-seastar,duarten/scylla,dwdm/scylla,leejir/seastar,guiquanz/scylla,chunshengster/seastar,raphaelsc/seastar,phonkee/scylla,rentongzhang/scylla,glommer/scylla,capturePointer/scylla,sjperkins/seastar,jonathanleang/seastar,norcimo5/seastar,dreamsxin/seastar,syuu1228/seastar,kjniemi/scylla,capturePointer/scylla,hongliangzhao/seastar,cloudius-systems/seastar,dwdm/scylla,guiquanz/scylla,sjperkins/seastar,phonkee/scylla,bowlofstew/seastar,dwdm/seastar,gwicke/scylla,capturePointer/scylla,wildinto/seastar,printedheart/seastar,slivne/seastar,avikivity/scylla,bzero/seastar,kjniemi/scylla,bzero/seastar,victorbriz/scylla,dwdm/seastar,avikivity/scylla,syuu1228/seastar,raphaelsc/scylla,scylladb/scylla-seastar,respu/scylla