Modules API: blocked client free callback modified to get a context.

Note that this was an experimental API that can only be enabled with
REIDSMODULE_EXPERIMENTAL_API, so it is subject to change until its
promoted to stable API. Sorry for the breakage, it is trivial to
resolve btw. This change will not be back ported to Redis 4.0.

src/modules/hellocluster.c

@@ -30,6 +30,7 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#define REDISMODULE_EXPERIMENTAL_API
 #include "../redismodule.h"
 #include <stdio.h>
 #include <stdlib.h>