PIG-3661: Piggybank AvroStorage fails if used in more than one load or store statement (rohini)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1561213 13f79535-47bb-0310-9956-ffa450edef68
PIG-3463 Add missing test file (aniket486)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1561258 13f79535-47bb-0310-9956-ffa450edef68
PIG-2207: Add missing null check for msgMap

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1561259 13f79535-47bb-0310-9956-ffa450edef68