Remove LLRU from storage engine

[BohuTANG]

I think a storage engine doesn't need a internal LRU, what's about you?

Reason as follows:

1) Storage engine just do fast Write/Read(guarantee less I/Os), not sensitive the optimal of cache.

2) The external LRU(such put datas to Redis or others memory cache) is more flexible.

So, I removed the 'LRU' from 'unstable' branch.

Hope for any suggestions.

Thanks

BohuTANG