API: Notifications for Incoming STREAM Packets

[emschwartz]

Websocket endpoint on the API that sends out notifications about incoming STREAM packets with the following information:

• STREAM ID / Payment Pointer
• Sender Payment Pointer (if possible)
• Amount
• Timestamp
• Data from packet

Originally part of #128

[emschwartz]

We may actually want to leave the data out of the initial version of this API. The STREAM protocol is designed to ensure that the data is received in the correct order with no gaps, even if it is split across multiple packets. However, it's possible that the data will arrive out of order, and even possible that the sender could send two packets with some bytes overlapping. It would be a pain to handle these cases in the client to this API so it would be better to use a stateful STREAM receiver when we want to use data sent over STREAM.

[emschwartz]

Resolved by #307