RFC: Replace GooMutex by std::recursive_mutex

Since we are a C++14 code base now, there is no need for our own abstraction for mutexes. Hence the first commit removes the GooMutex wrapper replacing it with direct usage of std::recursive_mutex. For simple reference counts like that used by Array, Dict and Stream, it avoids using the mutexes and uses std::atomic_int instead.

The second commit does remove the conditional define MULTITHREADED, i.e. always multithreaded to emphasize that threading concerns must always be taken into account and simplify the code base by reducing the number of preprocessor combinations to consider.