dict.c: added optional callback to dictEmpty().

Redis hash table implementation has many non-blocking features like
incremental rehashing, however while deleting a large hash table there
was no way to have a callback called to do some incremental work.

This commit adds this support, as an optiona callback argument to
dictEmpty() that is currently called at a fixed interval (one time every
65k deletions).

src/t_list.c

@@ -837,7 +837,7 @@ void unblockClientWaitingData(redisClient *c) {
     dictReleaseIterator(di);
 
     /* Cleanup the client structure */
-    dictEmpty(c->bpop.keys);
+    dictEmpty(c->bpop.keys,NULL);
     if (c->bpop.target) {
         decrRefCount(c->bpop.target);
         c->bpop.target = NULL;