ZOOKEEPER-1552. Enable sync request processor in Observer (thawan, fpj)

git-svn-id: https://svn.apache.org/repos/asf/zookeeper/branches/branch-3.4@1527771 13f79535-47bb-0310-9956-ffa450edef68
ZOOKEEPER-1758. Add documentation for zookeeper.observer.syncEnabled flag (thawan, fpj via thawan)

git-svn-id: https://svn.apache.org/repos/asf/zookeeper/branches/branch-3.4@1527778 13f79535-47bb-0310-9956-ffa450edef68