Separate out DOMDataTree{Read,Write}Transactions

Separate out DOMDataTree{Read,Write}Transactions

DOMDataTreeReadWriteTransaction.close() does not make sense
in presence of DOMDataTreeReadWriteTransaction.cancel().

Split out both operation sets, so ReadWriteTransaction can eventually
become a WriteTransaction which also has ReadOperations.

Make DOMDataTreeReadWriteTransaction.close() a default method, which
defers processing to cancel() and remove its other implementations
-- some of which were completely broken.

Change-Id: Ib45e3e0d17ae2be591915fa618d5b468ec0c89e7
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>