BUG-7464: Fix original porting damage in conditional operations

BUG-7464: Fix original porting damage in conditional operations

Scala version of INode returns a correct empty Option
in case an LNode does not contain the expected key, whereas
this implementation happily falls through to a RuntimeException,
this this porting damage, adding an appropriate comment.

Removal of values through Map.remove(Object, Object) was
inconsistent in that the SNode case uses v.equals() whereas
the LNode case uses identity check. This is inconsistent
and runs contrary to the interface specitication, which calls
for using Objects.equals() -- which boils down to v.equals()
just as the SNode case does.

Change-Id: I33dd986e59211aa4c0be3648ae35e2818ebce8ad
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>