Implement introspect for show routing instance summary

This will be useful for debugging when there's a large number of
instances. The normal show routing instance introspect output has
too much information about table membership, making it difficult
to find the instance of interest.

Change-Id: Ibbd4be2196cf6f5a4b359a6fd3ceecc4b8b5e050
Merge "Implement introspect for show routing instance summary"