Util/TriState: new enum type
AirspacePolygon: lazy convex check

Convert AbstractAirspace::m_is_convex to a TriState and run the
GrahamScan on the first call to GetClearance().  Nobody else was
interested in the "m_is_convex" call, and GetClearance() has to do the
GrahamScan again anyway.
SearchPointVector: remove unused method IsConvex()