CAY-1868 Select contention with multiple contexts

porting from trunk

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/branches/STABLE-3.1@1553805 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/branches/STABLE-3.1@1553806 13f79535-47bb-0310-9956-ffa450edef68