GORA-211 thread safety: fix java.lang.NullPointerException - synchronize on mutator

git-svn-id: https://svn.apache.org/repos/asf/gora/trunk@1455955 13f79535-47bb-0310-9956-ffa450edef68
merge GORA-211 #1455955

git-svn-id: https://svn.apache.org/repos/asf/gora/branches/GORA_174@1456004 13f79535-47bb-0310-9956-ffa450edef68