Correct AbstractDataObjectModification memoization

Correct AbstractDataObjectModification memoization

Memoization here is not nice where nulls are concerned: if the result is
null, we end up round-tripping to the codec. Fix that by explicitly
masking null values with a sentinel object.

While we are at it, also relax access: instead of volatile read/writes,
use getAcquire()/setRelease() mechanics. Since the resulting DataObjects
are presumably previous, the set side is actually a CAS, reusing any
results of concurrent computation.

Change-Id: I77285e0842588b1882ef2eb09677d96395911a85
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>