Distance Check Approximation

[willhuff0]

I've roughly ported the distance check implementation from Prox.

It adds an equirectangular approximation for checking distances less than 1.5 km. Above which, the accuracy begins to drift significantly. If we ever need to check larger distances, the haversine distance equation (previous default) will be used.

This saves multiple resource intensive sqrt and trig operations reducing load on the server without loosing functionality.

[h1divp]

Thank you, this simplifies the affected code and the distance checking is much more accurate. Perhaps if we do set very large radii for messages then this will be incredibly helpful. The math work is also always appreciated.

I am accepting this PR, but note that I have not fully tested this since we still don't have thorough good enough checks for how accurate the message communication is with respect to geography. I just created issue #306 to address this.