Inform about function duplication.
    
    sss_mc_set_recycled is a static function, that should not
    be used outside nsssrv_mmap_cache.c. The sss_cache tool
    is an exception, because in the case when sssd is not running,
    sss_cache must invalidate the memory cache file. That is why
    sss_mc_set_recycled was copied to the tools_mc_util.c
    (as helper function for sss_memcache_invalidate function).
    It was duplicated to allow this function to remain static
    (and invisible to any .h files), so that it is not used anywhere else.
    Wrong usage of this function might cause race conditions and corrupt
    the cache.
    
    I'll add comments about the duplication to the code.