[openstreetmap/openstreetmap-website] Tiles at z17 not rendering properly (#1420)

[Elliott Plack]

I have observed that the tiles at z17 are not rendering with changes to the map. Their status date is **Sat Jan 01 00:00:00 2000**, which seems like an error. This does not affect z18-19 or 16-lower.

## Steps to reproduce
Tested on Google Chrome Version 55.0.2883.87 m (64-bit)

1. Navigate to an area with some recent edits, e.g. [here](http://www.openstreetmap.org/#map=16/39.5374/-76.6368). _Note that this is z16_.
2. [Disable cache](http://stackoverflow.com/a/7000899/2105596) for testing purposes
3. Force empty cache and reload (see Fig. A below). This will ensure that the latest non-cached tiles are loaded.
4. Zoom in a step to z17 and note the odd green notch in the park multipolygon. I fixed this over the weekend. Repeat step 3 to reload. When I tested this 31 Jan 2017 1511UTC, Fig. B is how z16 looked, and Fig. C is how z17 looked.
5. Try forcing a _dirty_ status on one of the z17 tiles. Look in the dev tools _Network_ tab for a PNG that should be updated, e.g. [49840](http://c.tile.openstreetmap.org/17/37633/49840.png).
6. add a `/dirty` to the end of the tile URL, i.e. `http://c.tile.openstreetmap.org/17/37633/49840.png/dirty`. You should see a message, "Tile submitted for rendering"
7. Wait a few minutes for good measure.
8. Repeat steps 2, 3, and 4 and note that there is no change.

## Further analysis
I was showing a newly mapped woods trail to someone unfamiliar with OSM yesterday and noticed that the tiles were acting up on a computer browser that had never launched the OSM website.

I [checked the tile status](http://c.tile.openstreetmap.org/17/37633/49840.png/status) for the affected tiles, e.g. 49840, and noticed that the last rendered date is always **`Sat Jan 01 00:00:00 2000`**

## Screenshots
Figure A (reload)
![image](https://cloud.githubusercontent.com/assets/1827881/22470008/d7fda078-e79b-11e6-8ee1-de807e073867.png)

Figure B (z16)
![chrome_2017-01-31_10-07-09](https://cloud.githubusercontent.com/assets/1827881/22470484/7cdf6cba-e79d-11e6-91ce-e349330ef6c7.png)

Figure C (z17)
![chrome_2017-01-31_10-08-52](https://cloud.githubusercontent.com/assets/1827881/22470487/80998bf6-e79d-11e6-907b-e43d816e761e.png)

Figure D (tile 49840 status)
![image](https://cloud.githubusercontent.com/assets/1827881/22471168/8ec59ff6-e79f-11e6-95f9-64ab609ed4c3.png)



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