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

[alex85k]

The tests passes with only one lock_guard. Strange result is that 1 processes is faster that 2 or 4 in that case (this time bulgaria-latest.osm.pbf was used). Time of processing pending ways increases from 34 to 43 s (1->2 processes).

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