File::exists() that file is actual file.

Add tests.

Change-Id: I39c0d1bc171cfa71707ef1e3b9987e5fd3a34cb3
Refactor DB DataIO.

Change-Id: Ib425e9e0287d506c68d8ef12c99b8a49fd5eaa57
Extend Node test.

Change-Id: I26ac1f973bb6012a7dc9dee595e646417c79769f
Remove comment from test.

Change-Id: Id9b72678d0dc6579d7cc5109275bcf3579d6d1e3