memcache: switch cache to use boost::intrusive::unordered_set
memcache: switch cache to use boost::intrusive::unordered_set

This will allow us to have LRU evictions and removal of expired items
which don't involve key lookups.

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