OAK-1242 Weak reference targets added after the reference property are not indexed

- also fixes: OAK-1257 Adding a new reference to an existing MV property deletes the back references of the old ones

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1547468 13f79535-47bb-0310-9956-ffa450edef68