core: provide "void" promise specialization
core: provide "void" promise specialization

Makes it easier to write generic code.

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