util: add reference_wrapper<>
util: add reference_wrapper<>

seastar::reference_wrapper<> is a construct very similar to
std::reference_wrapper<> except that it doesn't allow implicit construction
from references, but requires use of seastar::ref. This makes it possible to
force passing objects by lvalue to be more explicit.

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