future: Correct a few hypercorrect uses of std::forward
future: Correct a few hypercorrect uses of std::forward

In these cases we were using std::forward in places where the argument
is not a "universal reference", but just a class (lambda) member.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200622170755.171578-2-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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