This is a follow up commit for rB309c919ee9.
Clearing hash tables is now parallelized as well. Surprisingly, most of
the time is actually spent in `free` (a couple of milliseconds per call
in my test).
Benchmark of individual functions:
reserve_hash_maps: 17%
add_polygon_edges_to_hash_maps: 49%
serialize_and_initialize_deduplicated_edges: 12%
update_edge_indices_in_poly_loops: 14%
clear_hash_tables: 5%
Jacques Lucke
2f339eb745