Create a solution for ReceptionStatus handling

[wcoenraads]

When receiving a message, there is currently the UnconfirmedMessage type in the library to make sure you send a ReceptionStatus back. I would like to also create a utility type to help users handle reception statuses that come in response to messages they've sent.

What it would need to do:

• Keep track of sent messages
• When an OK reception status comes in, tick it off as confirmed in the list of sent messages
• When an error reception status comes in, notify the user. In the case that e.g. a measurement message was returned with an error reception status, not too much needs to happen (though you can retry sending it). But in the case that e.g. a system description message got an error reception status, that indicates a bigger problem - at that point it's not clear whether sensible control can happen
• Provide a mechanism that notifies the library user when a sent message has not received a reception status for a long time - this can indicate a problem with the connection, or that the other side of the connection has a crappy S2 implementation