Revert "future-util: reduce allocations and continuations in parallel_for_each"
Revert "future-util: reduce allocations and continuations in parallel_for_each"

This reverts commit 951effa90ed47040827d0da17c7ee4045036249d. It fails in distributed_test
in debug mode.

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