Use stale TileJSON data

[kkaefer]

With #2617 in place, we now get stale data from the cache. Currently, stale TileJSON data loaded as part of loading a Source aren't used, but we should change this.

• 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. This is not a typical case however, so we could implement this with drastic action.

Things that could change in a TileJSON:

• Tile endpoint: We need to reload all tiles to make sure we load the right data
• Tile size: We need to recalculate the tiles we need to load because we might have to load tiles for a different zoom level
• Min/Max zoom: We need to recalculate what tiles to load, if we have tiles loaded that are outside the new zoom range
• Attribution: We need to notify the embedding application that this changed (Derive attribution from sources in current style #2723)
• Center/bounds: We're not using these values currently

[kkaefer]

Implementation happens as part of #2721