BUG-509: fix ModifiedNode locking

This removes the 'sealed' flag from ModifiedNode, as it is not a
publicly-visible object. Also drop the synchronized blocks and
reintroduce them in InMemoryDataTreeModification, which is the user
entrypoint.

Change-Id: I6c8be61701134bd8f645b8d4f0a8ef3ae218f0ac
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-509: fix ModifiedNode locking"