kdu_cache::acquire_lock

This function, together with release_lock, is used to guard access to the cached data. The implementation here does nothing at all, but a derived class may override these virtual functions to provide synchronization for multi-threaded environments. In particular, if add_to_databin or any of the other cache management functions might is to be called from a different thread to that which reads data from the cache, the present function should lock, a mutex and the release_lock function should release it.