Optimize Geometry(TopoGeometry) for polygons

[strk]

Current TopoGeometry::Geometry cast makes an ST_Union of the geometry obtained with each face. Instead, it could apply the BuildArea? directly on the set of edges referencing all the faces composing the TopoGeometry.

In addition to skip the union and passing everything to BuildArea? we'd have another advantage in that we could skip edges having one of the TopoGeometry faces on _both_ sides as those edges would dissolve in the output anyway.

[strk]

So, at a closer look it turned out the bottleneck was on fetching edges composing faces rather than polygonizing them. Creating indexes on "left_face" and "right_face" speeded things up by a factor of 10. r6822 adds index creation to CreateTopology?.