future: fix parallel_for_each returning to quickly on error
future: fix parallel_for_each returning to quickly on error

parallel_for_each() returned as soon as it detected an exception, rather
than waiting for all fibers to complete.  This can cause all manners of
resources leaks and races.

Fix by using then_wrapped() and a counter to make sure every function
completed.

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