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

[alex85k]

The usual tests seem to pass. (actually, there are same 3 failing muti-backend tests with master branch, but it is for another ticket).

However, the Python regression test (which is multithreaded, I guess), breaks each osm2pgsql with some "heap memory corruption" error deep inside geos call. Will investigate more and put the details here.

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https://github.com/openstreetmap/osm2pgsql/pull/498#issuecomment-154835496
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