Improve SnapshotBackedReadTransaction.close()

This improves visibility of close() operation, resulting in improved
close/read collision detection.

Change-Id: I7d3367c4d669a5c3dcd3f5c6583ce64b7690f1d9
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Improve SnapshotBackedReadTransaction.close()"