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/sentinel.c

@@ -393,7 +393,7 @@ void initSentinel(void) {
 
     /* Remove usual Redis commands from the command table, then just add
      * the SENTINEL command. */
-    dictEmpty(server.commands);
+    dictEmpty(server.commands,NULL);
     for (j = 0; j < sizeof(sentinelcmds)/sizeof(sentinelcmds[0]); j++) {
         int retval;
         struct redisCommand *cmd = sentinelcmds+j;