ViewResponse should implement Send

[cmbrandenburg]

ViewRow contains an Rc<DatabaseName> as an optimization. However, this has the effect of making the ViewRow implement !Send. See #19 for more commentary.

I.e., the follow snippet causes a compile-time error:

fn f<T: Send>(_: T) {}
f(ViewResponse::Reduced(ReducedView {
    update_seq: None,
    value: 42,
}));