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

[alex85k]

I am afraid Python regression tests still do not work. There are access violation errors in async call:
![image](https://cloud.githubusercontent.com/assets/4124648/11021395/cc64fcc6-8661-11e5-9ae6-f0705ba7b1ec.png)


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