Improve/cleanup the multi-threading support

The pthreads based multi-threading code should be improved (IMHO). I see two options:

1. keep the internal 'explicit' interface more or less like it is but switch from pthreads to c++11 threads.
2. remove the 'explicit' interface, which should simplify the code significantly and switch to OpenMP where applicable.

I would strongly prefer the second option as is would reduce the complexity of the code and thereby make it a lot easier or even make it possible for the first time to add it where it currently is not. E.g. CR2 files could potentially be decoded in parallel when they are composed of multiple strips. I have not looked into this properly, just wanted to bring up the idea and get feedback from others.

If I see it correctly darktable is also embracing the OpenMP methodology. With recent Apple/Clang compilers supporting it as well platform support should be sufficient.

Begin by locating the pthreads-based multithreading implementation and reviewing how its explicit interface is used. Compare the proposed C++11 threads and OpenMP approaches, including whether parallel decoding could extend to CR2 files composed of multiple strips. The issue does not name files or tests, and done requires a settled design and implementation direction.