This patch removes the MY_ATOMIC_MODE_RWLOCKS atomics implementation
based on mutexes. This means that native atomics support is now required.

The patch simplifies our atomics API, by removing all my_atomic_rwlock_* macros
as well as my_atomic_rwlock_t.

The patch also removes the MY_ATOMIC_MODE_RWLOCKS and 
WITH_ATOMIC_LOCKS CMake parameters.