The RecursiveMutex class in mutex.h is unused in the driver, but causes the threadlocal.h header to be needed as a user visible include which in turn would require the configuration #define's for threadlocal.h to be part of a future config.h

Since RecursiveMutex is unused, we should just remove it from mutex.h in our tree.

Longer term, we should consider whether we want to remove almost all of mutex.h and just standardize on boost::mutex in the driver. The hand rolled mutex classes are almost certainly not advantageous in the driver.