[Tile-serving] Results on speed changes from .style files

[Paul Norman]

I completed tests on the impacts of a smaller osm2pgsql .style file 
on rendering speed. As expected, it increases the rendering speed, 
but it also does so by more than the percent reduction in database 
size.

Eliminating columns not used in osm-carto decreased DB size by 0.28% 
and increased rendering speed by 2.35%

More details at
http://www.paulnorman.ca/blog/2014/02/osm2pgsql-style-file-size/

Andy, do you think it's worth committing
https://github.com/pnorman/openstreetmap-carto/commit/93534f? 
On the one hand, they're not used, on the other, I know we have requests 
to use some of those tags, like public_transport, and it would feel odd 
to remove them, then potentially add them back later.