Added new implementation of the preparedLRU cache.

Hi Guys,

This is my second attempt at resolving issue #184. I took the comments from my last PR #187 and implemented a new version of the LRU cache for prepared statements using the ```github.com/golang/groupcache/lru``` package for guidance.

I implemented some integration tests to reproduce the issue this PR is trying to resolve. Also I didn't go to crazy with the integration tests because all queries are prepared and so the code is accessed many times with existing tests.

Hopefully this PR hits closer to the type of LRU implementation the team is looking for. Also I updated my [benchmark](https://github.com/phillipCouto/lrutest) to compare the LRU code in the PR against the lru package from groupcache and the PR code is out performing the lru package.