future: Erase types a bit more in then_wrapped
future: Erase types a bit more in then_wrapped

We were instantiating then_wrapped_common with the return type of the
original function, not the noncopyable_function.

This avoids instantiating then_wrapped_common with both int and
future<int> for instance.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200318020355.2117429-2-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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