Backport REPO-469: Add support for unlocking nodes by users with lock management privileges

We have several places in our code base now where we use JCR locking. But we have no way to allow administrators to manage those locks. Although it doesn't work out of the box, Jackrabbit provides extension points to implement such functionality. We need our own LockManager that allows users with lock management privileges to unlock a node.
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Check the possibility of backporting REPO-469 to 7.7