[PROPOSAL]Refine the implementation of LRUCache

[孙 岚]

Hi all,

         I found the LRUCache that Groovy is using is not efficient enough 
because it is based on Collections.synchronizeMap, the LRUCache is used by some 
important classes(e.g. ProxyGenerator, Closure), so it can impact the 
performance somehow.

         I propose to replace the implementation with 
Caffeine(https://github.com/ben-manes/caffeine), which is a high performance 
caching library for Java 8 and licensed under APL2.

        Any thoughts?

Cheers,
Daniel.Sun