[Tile-serving] [openstreetmap/mod_tile] `ModTileMissingRequestTimeout 0` doesn't mean infinity (#237)

[Stephan]

You have two different scenarios to consider.

Scenario 1: you have on disk a cached rendered tile, but it is outdated and due to be rendered.
Scenario 2: you have a request for a tile which does not exist.

The option `ModTileRequestTimeout` deals with scenario 1. It controls how long mod_tile waits for the renderer until it gives up and serves the old cached copy.
For interactive maps typically you do not want the client to wait, so you tell mod_tile to return the old cached copy and do re-render the tile in the background. Mod_tile sends tile-requests along with a priority, so renderer can distinguish.
A typical setup is then  `ModTileRequestTimeout 0`.

In scenario 2, there is no cached copy available. Options are either to wait or to return a 404. The option `ModTileMissingRequestTimeout` controls this waiting time.

So in your first test it would wait 30 seconds for the renderer and then give up and return 404, as there is no alternative.
In your other test it returns immediately the tile from disk. Regardless whether it current or not (triggering a rendering in background if needed). For tiles not existing it would wait a very long time. Unless you have other timeouts happening, it would wait for the tile until it is rendered. 

To give some ball-park numbers: The osm-cartho style has roughly these rendering times on a i7-8700:

        style     zoom |      0       1       2       3       4       5       6       7       8       9      10      11      12      13      14      15      16      17      18      19      20
        ---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
        osm            |                                                   12.5s   14.3s   10.9s   10.1s   34.4s   16.3s    8.3s    3.9s   20.6s   21.0s   23.7s   24.7s   22.2s   17.3s   20.5s



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