BUG-7464: make LNodeEntry implement Map.Entry

Since LNodeEntry already contains a key/value mapping, it is
advantageous to make it also implement Map.Entry, simply
because that allows us to skip temporary object instantiation
when iterating over entries.

This could be done via subclassing SimpleImmutableEntry, but
that forces us to implement Serializable, which is not what
we want.

Change-Id: I3b0a2384dfbb9c2ed14dd6c9d66dfe91e883f97a
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>