util/closeable: hold _obj with reference_wrapper<>
util/closeable: hold _obj with reference_wrapper<>

so we don't copy the referenced object when calling the move
constructors of deferred_stop and deferred_close.

this should address the unexpected behavior introduced by
d1758e355e02a099bd7b20a8feec8057767430ca

Signed-off-by: Kefu Chai <cdc92d111bae0ed1ee3b3c8a1f849fcffc6b44f7@gmail.com>

syuu1228/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar