future: make when_all(Future... future) mode selective
future: make when_all(Future... future) mode selective

Currently we have one version of when_all(), that accepts a compile
type variable number of independently typed futures.  We wish to add more
overloads, but for that, we must ensure that this variant doesn't match them.

Accomplish this by requiring that the first argument be a future, and don't
accept any other type.

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