Refactor "nearby" sharing notifications

[toland]

We've found an oddity when two users are both sharing their location when nearby. We send notifications and set the nearby flag for pushing user locations when a user uploads their location. In the case where both users are sharing nearby, we already know whether the other user is nearby or not without them uploading another location data point. We should go ahead and send notifications and update the internal state without waiting for the other user to upload a location.

There is a chance of a race condition here, and I'm not sure how to address that. Perhaps by pushing the updates through the other user's location processing handler.

[bernardd]

I wouldn't get too stressed about the race condition - it will resolve next time either user uploads a location, and the actual impact will be pretty minor if indeed it's noticeable at all. I think this is one of those cases where we can just cover our ears and go "lalalalala eventually consistent".

[toland]

That is the approach that I went with. We can deal with the race condition when and if it becomes a problem.