Caching

[kkaefer]

This is a caching master ticket:

• Harden glyph loading #308: Cache glyph PBFs indefinitely
• Cache tessellated polygon data #90: Cache tessellated polgyon data
• Overzoom cached/offline tiles #123: Load non-matching tiles from cache to cover up lack of data

[mb12]

@kkaefer
1.) Is it possible to add support for tessellation to the following mapbox projects?
https://github.com/mapbox/mapnik-vector-tile
https://github.com/mapnik/node-mapnik

[ansis]

The individual issues are still open. All of them are fairly separate in terms of implementation so I don't think we need this master ticket anymore