memory: switch page_list to use page indices instead of pointers
memory: switch page_list to use page indices instead of pointers

Two motivations:
1. reduce the size of struct page
2. allow relocating the page array; needed for dynamically changing
   the amount of memory supported.

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