Ignore lesser used old cache entries while invalidating cache entries in background read

Currently the Cache invalidation logic used in MongoDocumentStore check for cache consistency for all the entries present in the cache. With use of persistent cache its possible that pressure on backend cache would be reduced and some of the cache entries are not being accessed for long time.

Cache invalidation logic should take into account such access statistics and not perform consistency check for cached instance which are not accessed for some long time (10 mins?). Such cache entries should be directly discarded.

PS: Looking at [1] it appears that Guava cache does not enforces a global LRU eviction policy. The policy is maintained per segment table

[1] http://stackoverflow.com/questions/10236057/guava-cache-eviction-policy