Allow acking messages without a reference to publish

[einarmo]

The current system requires a reference to a Publish packet in order to send an ack for that packet. This is usually fine, but in our case we receive and process messages asynchronously, so we would like to be able to ack a message on a callback after processing it, potentially in a batch together with other messages. We could keep the message, but that seems unnecessary. You don't need the full message to send an ack, and the payload itself is probably better freed by that point.

A solution could be to either expose an ack_by_id(pkid, qos), or making a get_ack_packet(&Publish) -> Ack, and send_ack_packet(Ack), where Ack is some thin wrapper around Option<Request>.

I'd be happy to contribute this if it sounds reasonable.