Deprecate WriteTransaction.mergeParentStructure*()
These methods hark to our dark past. Most of their uses should be
superfluous with current semantics and they paper over important
lifecycle management questions. Furthermore there are corner cases for
which they cannot be made to work simply because of the semantics
involved.
Deprecate these method so their users are pointed out and can be
gradually migrated.
Change-Id: Ib85d42a9514b33fcdb1efb1ddb273b4c6ee0e554
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>