[Tile-serving] [osm2pgsql] Replace pthread and boost::thread with std::async (#498)

[alex85k]

Tested: some speedup is possible even with mutex. 625s -> 425s (pending ways 98s -> 56s). Used 4 threads on moble Intel i7 Haswell 4200u (2 real cores).

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/osm2pgsql/pull/498#issuecomment-155529121
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