Feature/pool ext variant pool ext (#433)
* added threads dependency to meson build because of pthread linkage problems (gcc version Debian 8.3.0-6)
* initial version of generic_pool_ext, pool_ext and variant_pool_ext (extended unit tests)
* format code
* fix test_call_if_and_not_valid_returning_void by moving SetupFixture into namespace (to ensure correct test setup)
Co-authored-by: Daniel B <daniel-brosche@users.noreply.github.com>
