Minor changes to RouteDistinguisher

Make BgpAttr::source_rd() return a const reference.
Rename RouteDistinguisher::null_rd to RouteDistinguisher::kZeroRd.
Rename RouteDistinguisher::IsNull to RouteDistinguisher::IsZero.

Change-Id: I2289ce4b5d4d323b0a99658976b4ce0c826b4c1f