add HighPriCache and LowPriCache interface to implement PrioritizedCache

[haoxiang47]

From this issue: tikv/titan#127
add two interface: NewHighPriCache and NewLowPriCache to implement PrioritizedCache

[yiwu-arbug]

@haoxiang47 Thanks for update! Comments:

1. With PrioritizedCache, what we want to achieve is having a single LRUCache as backend, but expose two wrapper for it. When using the HighPriCache wrapper, on insert cache item, always override priority to high; vice-versa for LowPriCache. It doesn't not change underlying high-pri-pool-ratio.
2. Can you move the change to titan repo (github.com/tikv/titan)? We would like to keep rocksdb change at minimum.

[haoxiang47]

ok, I will update my code:

1. from the lru_cache.cc, I find that only in high_pri_pool_ratio_ > 0 && (e->IsHighPri() || e->HasHit() the element can insert to the high priority, so if we do not change the high_pri_pool_ratio_, maybe we can not always insert into the high lru list? or we just want to add a wrapper that all element from this wapper will be overwrite the priority, but do not change the cache origin logic?
2. yes I will move it to the titan

[yiwu-arbug]

or we just want to add a wrapper that all element from this wapper will be overwrite the priority, but do not change the cache origin logic?

This is what I want to do.

[yiwu-arbug]

Closing in favor of tikv/titan#142