BUG-7464: Hide INode.gen field

This field is accessed only from TrieMap when initializing an operation
on a root node. Hide the field by exposing generation-less package-visible
methods, which pass the generation field to INode-private methods.

Change-Id: Ibf281c5975fe96d2cd2b1d58e93203a806932b57
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>