Improve duplicate update detection.

Handle the case where the combination of the AdvertiseSList and the
UpdateInfoSList in the RouteUpdate contain the same information as
the UpdateInfoSList from the export policy results.  We previously
used to do a simple comparison of the UpdateInfoSLists.
Convert static functions to methods for RouteState and RouteUpdate.
Further improvements in the duplicate detection code.

Handle the special case where the given UpdateInfoSList is empty.
Implement new logic that's easier to follow for the other cases.
Merge pull request #114 from Juniper/nsheth_duplicate_update

Improve duplicate update detection.