This pr can get two performance benefits:
1. Stop redundant initialization when most robj objects are created
2. LRU_CLOCK will no longer be called in io threads, so we can avoid the `atomicGet`
Another code optimization:
deleted the redundant judgment in dbSetValue, no matter in LFU or LRU, the lru field inold
robj is always the freshest (it is always updated in lookupkey), so we don't need to judge if in LFU
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