make connection phy async

[barbibulle]

As reported in #647 it isn't safe to delay the emission of the connection even until we have asynchronously read the connection PHY from the controller. Unfortunately, the controller doesn't provide the PHY information when a connection is made, it has to be requested with a command, which is an asynchronous sequence, and thus some data could be received between the time the PHY request is made and the response is received, in which case that data will arrive before the connection event is emitted.
The only clean solution here is that the Connection object can't have a synchronous phy property. It must be an async "property" (not really a property, because real properties can't be async, but an async def get_phy() method).
Also, as a consequence of this change, the connection_phy_update event must pass the PHY value as a parameter, so as not to required an un-necessary get_phy request from the listener to obtain the information.
This does change the API slightly, but it probably won't break too many users.