BUG-650: improve modification sealing performance

The seal operation is on a fast path, so performing synchronization is
not really advised. Downgrade synchronized blocks to volatile
modifications, which have better effects on the optimizer. We introduce
a SynchronizedDataTreeModification which the users can use when they
require a serialized thread-safe access.

Change-Id: Ieffcf1f5fef4d4df32fcdc30f992672d79dcf7bf
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-650: improve modification sealing performance"