core: introduce map_reduce() utility
core: introduce map_reduce() utility

It spawns async mapping action in parallel and reduces the results as
they come.

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