Harden glyph loading

[kkaefer]

• Make sure that we still draw a map, e.g. without labels, or with only some labels, if glyph PBFs fails to load. This likely requires implementing incremental parsing and updatable buckets (incremental parsing #163)
• Cache glyph images indefinitely
• Retry glyph loading when a request fails (e.g. due to shaky connection)

[hanchao]

The glyphs load timed out some time, the tile will not render. Will you consider using the system font #260

[0x10bb5b458] exception [5/26/12]... failed: [ERROR] failed to load glyphs (-1): The request timed out.

[incanus]

Cache glyph images indefinitely

Will there ever be a need to expire these if they get updated somehow on the server. Seems unlikely but we may want to key on date / etag / some sort of version info just in case.

[jfirebaugh]

We set a 10 day expiration for glyph PBFs on the new API endpoint.

[mikemorris]

etag expiration could be nice to implement at some point, but 10day seems fine to launch.

[kkaefer]

We are doing etag expiration now.

[jfirebaugh]

I don't think there's anything actionable here.