OAK-1941 - do not rely on connection.close rolling back the transaction

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1642299 13f79535-47bb-0310-9956-ffa450edef68
OAK-1941 - properly declare when we'll use a connection for reading only

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1642303 13f79535-47bb-0310-9956-ffa450edef68