Created lru cache using the groupcache/lru package.
Added missing cluster configuration option to adjust the size of the cache.
Moved preparedLRU definition to cluster.go as it seems more fitting. Reduced scope of locking for the prepared statement cache in the error handling code when executing batch statement. Improved the test cases.
Extended the TestPreparedEviction test to include testing the accuracy of the cache eviction process.
Refactored test code so a single test is used to validate the LRU functionality.
Changed test code from t.Errorf to t.Fatalf to fail fast.
Removed unnecessary continue statement in test.
Merge pull request #190 from phillipCouto/prepared_lru3

Added LRU Cache for Prepared statements to prevent memory leaks