Use streaming interface for price updates

Instead of pulling new prices every x second we should use the streaming api. See Oanda documentation AC:s

[ ] The system creates a streaming connection for all instruments at startup
[ ] The system reconnects in case of no data on the stream for more than ten seconds
[ ] The system can handle HTTP 429 error responses and re-connect
[ ] The system applies a exponential backoff when re-connecting

kumlien / fx2