Parallelize rock installation

[mikz]

Installing rocks is trivially parallelizable. We care only about nginx, so when ngx is available we can start a new thread. But it should be modular enough for adding more backends.

[mikz]

Using ngx.thread.spawn does not work because luarocks uses os.execute which is blocking and other threads are not getting scheduled. So it is still serialized.

See #6 for the prototype implementation.

One way to workaround this would be to reimplement fetching data by curl into cosockets.