Fix callback execution in redis search

`callback` should be passed to `reds.search` otherwise the execution will be overlapped. 

For example in the plugin `nodebb-plugin-dbsearch':

```javascript
db.searchRemove('topic', tid, function() {
	if (typeof title === 'string' && title.length) {
		db.searchIndex('topic', title, tid);
	}

	if (typeof callback === 'function') {
		callback();
	}
});
```

The actual execution is like this

```
1402387863.625553 [0 127.0.0.1:16660] "zrevrangebyscore" "nodebbtopicsearch:object:6" "+inf" "0"
1402387863.625891 [0 127.0.0.1:16660] "MULTI"
1402387863.626043 [0 127.0.0.1:16660] "zadd" "nodebbtopicsearch:word:123" "1" "6"
1402387863.626052 [0 127.0.0.1:16660] "zadd" "nodebbtopicsearch:object:6" "1" "123"
1402387863.626060 [0 127.0.0.1:16660] "zadd" "nodebbtopicsearch:word:TST" "1" "6"
1402387863.626065 [0 127.0.0.1:16660] "zadd" "nodebbtopicsearch:object:6" "1" "TST"
1402387863.626071 [0 127.0.0.1:16660] "zadd" "nodebbtopicsearch:word:AN0" "1" "6"
1402387863.626076 [0 127.0.0.1:16660] "zadd" "nodebbtopicsearch:object:6" "1" "AN0"
1402387863.626083 [0 127.0.0.1:16660] "zadd" "nodebbtopicsearch:word:TPK" "1" "6"
1402387863.626092 [0 127.0.0.1:16660] "zadd" "nodebbtopicsearch:object:6" "1" "TPK"
1402387863.626104 [0 127.0.0.1:16660] "zadd" "nodebbtopicsearch:word:ETTT" "1" "6"
1402387863.626116 [0 127.0.0.1:16660] "zadd" "nodebbtopicsearch:object:6" "1" "ETTT"
1402387863.626130 [0 127.0.0.1:16660] "EXEC"
1402387863.626253 [0 127.0.0.1:16660] "del" "topic:6:tags"
1402387863.626281 [0 127.0.0.1:16660] "MULTI"
1402387863.626515 [0 127.0.0.1:16660] "del" "nodebbtopicsearch:object:6"
1402387863.626524 [0 127.0.0.1:16660] "EXEC"
```

The key nodebbtopicsearch:object is added and then just being deleted again.