rgw: hold reloader using unique_ptr

rgw: hold reloader using unique_ptr

instead of using optional<> for holding reloader, use unique_ptr<>.
as `RGWRealmReloader` is neither
trivially_copy_{assignable,constructible} nor
is_trivially_move_{assignable, constructible}, because of the `Cond`
member variable. but Clang++ and libc++ still tries to rely on a
delgating copy constructor for constructing the
optional<RGWRealmReloader> instance even the optional object is
created `in_place`.

in this change, to workaround this issue, reloader is instead
constructed using make_unique<>

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 2494ddbef5ad20cdfccf59911fccadb6f9848da0)