several changes in cache cleanup handling:

*) now ngx_http_file_cache_cleanup() uses ngx_http_file_cache_free()
*) ngx_http_file_cache_free() interface has been changed to accept r->cache
   ngx_http_file_cache_cleanup() must use r->cache, but not r, because
   there can be several r->cache's during request processing, r->cache may
   be NULL at request finalising, etc.
*) test if updating request does not complete correctly