Vector read functions don't use command queue

I've noticed inconsistent behavior when reading values from vector using `.back()` routine. Digging it I've realized that essentially most of the vector read routines rely on the buffer_value class to read values from buffer and it uses some dummy command queue.
```
operator value_type() const
{
if(m_buffer.get()){
const context &context = m_buffer.get_context();
const device &device = context.get_device();
command_queue queue(context, device);

return detail::read_single_value(m_buffer, m_index / sizeof(T), queue);
}
else {
return m_value;
}
}
```

I suggest to add command queue argument to read routines, this way you can ensure that operations on the buffer have been performed before actually reading values from it.