branch to use label caching

[springmeyer]

If we can land http://trac.mapnik.org/ticket/870 (which is not working yet) then we should quickly create a branch (likely based off of unified) that attempts to reduce the # of layer queries down to the same # that mapserver uses.

[springmeyer]

@ajashton - I'm ready to test this asap on the benchmarking server - any progress on adapting the style to this patch? We're not planning on adding this patch to trunk mapnik atm, so we will need to all use the patch for the time being.

[ajashton]

https://github.com/mapbox/foss4g-benchmark-style/branches/deferred-labels