Return const refs to RouteDistinguisher and EthernetSegmentId

Automatic copy constructor and assignment operator do not work for
RouteDistinguisher and EthernetSegmentId.
Return const refs to these classes instead of returning by value.
The only exception is the static ::FromString method.

Change-Id: I1493d1ccf5dfeb8ef97b8d4c429c492cc61eddf5
Partial-Bug: 1399603