CAY-1868 Select contention with multiple contexts

we still need to remove the original synchronization

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1553803 13f79535-47bb-0310-9956-ffa450edef68
CAY-1868 Select contention with multiple contexts

release notes

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1553804 13f79535-47bb-0310-9956-ffa450edef68