Data race in conqueue

Affected versions of this crate unconditionally implemented Send/Sync for QueueSender<T>, allowing to send non-Send T to other threads by invoking (&QueueSender<T>).send().

This fails to prevent users from creating data races by sending types like Rc<T> or Arc<Cell<T>> to other threads, which can lead to memory corruption. The flaw was corrected in commit 1e462c3 by imposing T: Send to both Send/Sync impls for QueueSender<T>/QueueReceiver<T>.