Use stale Glyph PBFs

[kkaefer]

With #2617 in place, we now get stale data from the cache. Currently, stale Glyph data isn't used, but we should change this. Things to consider:

• The easy case is when the refreshed request returns a 304. In this case, we don't have to do anything.
• The hard case is when we get a different response. Glyphs that change may have different metrics. That means we'll have to reparse any layers that used glyphs from the updated glyph PBF
• The same is true for the Glyph Atlas: We need to update the Glyph stored in the atlas, and probably assign a different position.

On the other hand, glyphs that change dimensions are very rare and should not occur in practice. This could mean that we could just completely blow away the Glyph atlas, and all parsed buckets and do a complete reparse of all symbol buckets.

[kkaefer]

Implemented as part of #2721.