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

[alex85k]

By the way, one possible threading isuue:
![image](https://cloud.githubusercontent.com/assets/4124648/11085943/f35beb24-886d-11e5-9a70-c0397226de7f.png)
std::setlocale is set and restored from multiple threads.

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https://github.com/openstreetmap/osm2pgsql/pull/498#issuecomment-155690080
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