core: Use futurize<> to unify future::then() specializations
core: Use futurize<> to unify future::then() specializations

As a side benefit, returning non-void non-future is now allowed:

 future<int> x() {
     return later().then([] {
         return 3;
     });
 }

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