aioble/peripheral: Handle immediate disconnect.

[andrewleech]

If the client disconnects immediately after connection, the irq can be run before the initial connect handler has finished.

[puppet13th]

+1

[jimmo]

Thanks @andrewleech

I think maybe this could just be simplified by having DeviceConnection::__init__ always unconditionally create the event. I'm not sure I can see any code paths now that don't end up initialising self._event anyway. i.e. construction of a DeviceConnection for either a central or a peripheral will always end up needing to initialise the event. So we'll always have to do this allocation regardless, might as well do it up-front.

Then we can get rid of all the various places that create-or-reuse the event...less code overall.