Use user-provided osmium::thread::Pool instance
Use user-provided osmium::thread::Pool instance

Advantages:
* Allows to use Pool configured with user-preferred number of threads.
* If libosmium used in a shared library, this helps to prevent potential
  deadlock during Pool shutdown at (after) shared library unloading.

See https://github.com/osmcode/libosmium/pull/214

Requires libosmium 2.13.0

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