future-util: relax concept requirements for do_for_each()
future-util: relax concept requirements for do_for_each()

do_for_each() accepts a function object that returns either a future<>
or void. The concept incorrectly rejected the latter.

Message-Id: <4e0af5b79b7e946063824985ceec8593a5392350@scylladb.com>

scylladb/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,dreamsxin/seastar,syuu1228/seastar