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

[Roland Bosa]

@hummeltech I [rewrote](https://github.com/openstreetmap/mod_tile/commit/a0a6586a7a35a14606542dcf070a2e40e66a605c?diff=unified&w=1) the shutdown code a little and hooked it into the signal handler. I have only done minimal testing on this, but feel free to take a look at "[the good parts](https://github.com/openstreetmap/mod_tile/compare/master...rolandbosa:mod_tile:clean_shutdown)". 

I do get a message from `jemalloc` about leaks now, but I don't recognize any of the symbols:

```
...
** INFO: 13:19:04.695: Render thread 7/8 exited.
** INFO: 13:19:04.695: Destroying request queue...
** INFO: 13:19:04.695: Request queue destroyed.
<jemalloc>: Leak approximation summary: ~2488287 bytes, ~5 objects, >= 3 contexts
<jemalloc>: Run jeprof on dump output for leak detail

❯ jeprof --text src/renderd jeprof.11418.0.f.heap 
Using local file src/renderd.
Using local file jeprof.11418.0.f.heap.
Total: 2.6 MB
     1.6  61.4%  61.4%      2.1  80.7% std::vector::_M_realloc_insert
     0.5  19.3%  80.7%      0.5  19.3% get_self_exe_path
     0.5  19.3% 100.0%      0.5  19.3% __trans_list_add
     0.0   0.0% 100.0%      0.5  19.3% Option::Option
     0.0   0.0% 100.0%      2.1  80.7% SubProcessTimed::~SubProcessTimed
     0.0   0.0% 100.0%      2.6 100.0% _dl_init
     0.0   0.0% 100.0%      2.6 100.0% _dl_start_user
     0.0   0.0% 100.0%      2.1  80.7% build_options
     0.0   0.0% 100.0%      2.6 100.0% call_init
     0.0   0.0% 100.0%      0.5  19.3% flnl_result_alloc
     0.0   0.0% 100.0%      2.1  80.7% fmt::v7::detail::error_handler::on_error
```

The above output is from spinning up `renderd` and immediately asking it to exit (no tile rendering). The output below has force-rendered all tiles at zoom level 4 and 6 and shows the same leaks: I think the rendering code itself seems to be leak-free if allowed to terminate.

```
** INFO: 14:28:32.856: Render thread 7/8 exited.
** INFO: 14:28:32.856: Destroying request queue...
** INFO: 14:28:32.856: Request queue destroyed.
<jemalloc>: Leak approximation summary: ~2488287 bytes, ~5 objects, >= 3 contexts
<jemalloc>: Run jeprof on dump output for leak detail

❯ jeprof --text src/renderd jeprof.17275.0.f.heap 
Using local file src/renderd.
Using local file jeprof.17275.0.f.heap.
Total: 2.6 MB
     1.6  61.4%  61.4%      2.1  80.7% std::vector::_M_realloc_insert
     0.5  19.3%  80.7%      0.5  19.3% get_self_exe_path
     0.5  19.3% 100.0%      0.5  19.3% __trans_list_add
     0.0   0.0% 100.0%      0.5  19.3% Option::Option
     0.0   0.0% 100.0%      2.1  80.7% SubProcessTimed::~SubProcessTimed
     0.0   0.0% 100.0%      2.6 100.0% _dl_init
     0.0   0.0% 100.0%      2.6 100.0% _dl_start_user
     0.0   0.0% 100.0%      2.1  80.7% build_options
     0.0   0.0% 100.0%      2.6 100.0% call_init
     0.0   0.0% 100.0%      0.5  19.3% flnl_result_alloc
     0.0   0.0% 100.0%      2.1  80.7% fmt::v7::detail::error_handler::on_error
```


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