Clean up pooling meta-controllers

This patch removes the virtually unused RoutingController class, and has
each meta-controller inherit from its respective base storage class
instead.

It also adds get_{message,claim,queue}_controller methods to Catalog,
which allows the removal of some duplicated code in the meta-controller
methods.

Change-Id: I26b095317e4aa16c23ee14f2465c71e7e6bd0de4
Merge "Clean up pooling meta-controllers"