BUG-509: introduce Version concept

This patch introduces the concept of a Version, which is disconnected
from any ordinal number. This is useful for saving memory, as the
versioning of TreeNodes needs to only detect version mismatches and not
which version is historically newer.

Change-Id: I625d9d945ff3fd416dd82bb3f1ff4268f2001e5a
Signed-off-by: Robert Varga <rovarga@cisco.com>