Improve locality of tile backend requests

[pnorman]

With the previous CDN all users in a country tended to be routed to the same rendering server. Based on the assumption that users in a country tend to request the same metatiles, this meant that there was a higher chance of a metatile being fresh in the tile store than what we have now, which is weighted round-robin in Europe for high zoom tiles.

Currently we have two lower-end servers in Europe dedicated to zoom 0-12. Higher zooms go to a group of two high capacity and two middle capacity servers which are weighted round-robin.

The rest of the world is not an issue.

• The service must work with any one server failing, although may be degraded
• Software development is outside the scope of the OWG and sysadmins
• Selection needs to be done at the edge in VCL

[pnorman]

As an experiment I'm taking (x/8)%2 to assign requests to one of two groups in Europe. This seems to of had an immediate impact.

[pnorman]

The experiment seems to be working. It's good enough, given the constraints above.