replace pthreads & gcc specific threading and atomics with C++11 standard

As promised, GCC specific atomics and pthreads synchronization changed to standard C++11 With these changes erocksdb NIF builds on Windows (with manual "help" during the build process).

I did limited testing (on Windows) and erocksdb seems to be working as expected.

I also fixed a silly mistake and now your CI tests also pass