future: make futurize::apply() more flexible
future: make futurize::apply() more flexible

Allow it to accept functions that already return a future, instead of
being limited to functions that return the undelying type.

anzihenry/seastar,dreamsxin/seastar,tempbottle/seastar,xtao/seastar,avikivity/seastar,bowlofstew/seastar,printedheart/seastar,leejir/seastar,xtao/seastar,stamhe/seastar,mixja/seastar,avikivity/seastar,norcimo5/seastar,shyamalschandra/seastar,acbellini/seastar,cloudius-systems/seastar,wildinto/seastar,jonathanleang/seastar,sjperkins/seastar,kjniemi/seastar,syuu1228/seastar,chunshengster/seastar,flashbuckets/seastar,acbellini/seastar,norcimo5/seastar,printedheart/seastar,dwdm/seastar,tempbottle/seastar,dreamsxin/seastar,linearregression/seastar,bowlofstew/seastar,leejir/seastar,xtao/seastar,stamhe/seastar,shyamalschandra/seastar,koolhazz/seastar,shyamalschandra/seastar,printedheart/seastar,koolhazz/seastar,norcimo5/seastar,acbellini/seastar,chunshengster/seastar,scylladb/seastar,dwdm/seastar,koolhazz/seastar,scylladb/scylla-seastar,mixja/seastar,flashbuckets/seastar,wildinto/seastar,sjperkins/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/scylla-seastar,stamhe/seastar,hongliangzhao/seastar,kjniemi/seastar,scylladb/scylla-seastar,scylladb/seastar,joerg84/seastar,dreamsxin/seastar,linearregression/seastar,syuu1228/seastar,bowlofstew/seastar,joerg84/seastar,anzihenry/seastar,joerg84/seastar,tempbottle/seastar,linearregression/seastar,chunshengster/seastar,leejir/seastar,kjniemi/seastar,mixja/seastar,cloudius-systems/seastar,hongliangzhao/seastar,sjperkins/seastar,jonathanleang/seastar,hongliangzhao/seastar,jonathanleang/seastar,wildinto/seastar,flashbuckets/seastar,avikivity/seastar,dwdm/seastar,anzihenry/seastar