Fix RouteEntry.{add,remove}Route()

Fix RouteEntry.{add,remove}Route()

This API is interfacing with MD-SAL, where values are boxed,
hence these should be propagate as they are, not forcibly unboxed
in LocRibWriter.

This improves non-addpath performance, as we are simply skipping
the unboxing operation, since we do not care at all about pathIds.

Also hide RouteKey constructor and its accessor, as everything
aside of it being an identifier (hence immutable) is a purely
package-private matter.

Change-Id: Ica7612ec5e6df8d07eb16294c1efa5fb0428f884
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>