[Tile-serving] osm-carto v2.6 to v2.8 speed compairson

[Paul Norman]

I switched from v2.6.0 to v2.8.0 of osm-carto on the benchmarks I'm doing,
and tested for any speed differences. The rendering rate did not have any 
statistically significant changes, and remained at 8.81 MT/s.

https://github.com/gravitystorm/openstreetmap-carto/compare/v2.6.0...v2.8.0

This actually surprised me slightly, because the reducing of the number of 
queries with the road cleanups (e.g. 533d7c8395) does speed up rendering, 
on the other hand, fixing some bugs required round line cap joins for all 
roads, which is also a marginal speed decrease.