PIG-3979: group all performance, garbage collection, and incremental aggregation

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/branch-0.14@1631268 13f79535-47bb-0310-9956-ffa450edef68
PIG-4230: Documentation fix: first nested foreach example is incomplete

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/branch-0.14@1631270 13f79535-47bb-0310-9956-ffa450edef68