PIG-3654: Add class cache to PigContext

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1560468 13f79535-47bb-0310-9956-ffa450edef68
PIG-3654: Add class cache to PigContext

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1560469 13f79535-47bb-0310-9956-ffa450edef68