GORA-210 thread safety: fix java.util.ConcurrentModificationException

git-svn-id: https://svn.apache.org/repos/asf/gora/trunk@1455488 13f79535-47bb-0310-9956-ffa450edef68
merge GORA-210

git-svn-id: https://svn.apache.org/repos/asf/gora/branches/GORA_174@1455494 13f79535-47bb-0310-9956-ffa450edef68