[Tile-serving] [openstreetmap/mod_tile] Examine memory usage for potential leaks (Issue #445)

[Hummeltech]

> Here's a quick and dirty visualization of the respective memory footprints. All tests are done consecutively, but plotted on the same (relative) time axis.
> 
> Conclusions so far:
> 
>     * `jemalloc` seems the only one that returns substantial memory to the system
> 
>     * `tcmalloc` seems to not ever return anything, maybe additional configuration is required?
> 
>     * `glib` does have a dip here and there - are those just "lucky" allocations that reduced the top-of-heap?
> 
> 
> Timeline: 0s: start `renderd` - it loads the mapnik style file within ~10-15 seconds 30s: issue render request for all tiles at zoom level 6 45s-100s: database is churning like crazy, `renderd` threads are mostly idle 100-220s: `renderd` threads are using up CPU and memory 220s-250s: render_list exits, all rendering is done 300s: benchmark stops
> 
> ![image](https://private-user-images.githubusercontent.com/1264759/339883361-e6e8359d-dacc-4c21-9c1c-e61c95f71ecb.png?jwt=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.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._gtdE3loVLbMEc6hDTuquneUvKXkXDs59A0meyiXZCU)

@rolandbosa IIRC, some of `mod_tile`'s dependencies are likely the cause of the high memory consumption displayed in this chart. I will have to more closely at it again, but thank you very much for compiling this!

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