Replace LFU with LRU cache replacement policy

The LFU (least-frequently-used) replacement policy would encounter the
performance issue when the cache is not big enough to hold all
translated blocks (IRs). The LRU (least-recently-used) replacement
policy is more suitable for our needs. However, when the least-used
cache is dropped, the counter which is used to trigger JIT compilation
is going to be cleared.

To address this issue, we store the "live" cache in both list and hash
map, and manage the history of the replaced cache in hash map. Since the
out-of-use cache is added again, it can retrieve the previous counter.
However, the hash map might become very big if we don't trim the unused
history in it. We use the distance with the current counter and the last
accessed time to determine the history should be dropped or not.

Author: vacantron