Re-implement parallelization via `std::thread`

[atksh]

Previously parallelization was implemented by open-mp.

However, it is not able to compile with open-mp by cmake on mac and windows platform, and therefore we can't register it to PyPI via Travis CI w/ cibuildwheel with open-mp. So, there is no parallelization support now.

Since std::thread is a cross-platform multi-thread library, it is expected that we can compile it on any platforms (i.e, gcc, clang and visual studio). Plus, std::async looks to be good for parallelization with returning some deferrable values.

Now, there is little difference between batch_query and query methods; this parallelization would make batch_query much faster than single query again.