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

[Roland Bosa]

Hm, I can see this possibility for the fault: I tried to find a way to distinguish if a statistics thread was created or not by initializing the value of `stats_thread` with a "null value". Apparently this doesn't exist portably, instead one should use the value of `pthread_self()` and do a `pthreads_equal()` [comparison](https://stackoverflow.com/questions/6276939/is-there-an-invalid-pthread-t-id). However, according to [the man page](https://man7.org/linux/man-pages/man3/pthread_create.3.html#RETURN_VALUE) (and also as _qbert220_ points out), the value is undefined upon return, so a proper implementation should also re-initialize the variable upon creation failure (in addition to the initialization before checking if a statistics threads is requested at all). Why the creation would fail is a different question though...

In either case, the statistics thread is also sitting in a hard `sleep()` and that should be rewritten to use a timed condition sleep: sleep for the requested time OR when a exit has been requested. But since this was just testing code to figure out any post-mortem leaks, I chose the "Can't be bothered with this detail now"-way.

These days, `renderd` does not terminate cleanly (the signal handlers didn't work for me, they seem to be a mechanism for the render threads to indicate a catastrophy (e.g. style/font loading failed)). Wouldn't the proper way be to respond to a signal? I'd imagine that's what e.g. `systemd` would send to the process e.g. during a shutdown/reboot and nowadays it just gets a `kill` signal after some time of not responding to it...? How important would that be as a feature?

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