Implement introspect for show route summary

This is useful for debugging when there's a relatively large number
of routes spread over many tables. The normal show route introspect
output has too much information, making it dofficult to find table
of interest.

Further, after bug 1325652 gets fixed, normal show route introspect
won't show all tables anymore.

Change-Id: Ifc987e23094035d27b7ce79071d2c5ada52828b0
Related-Bug: 1325652