Add support for listing claimed messages (storage)

This patch modifies the default behavior of the message listing
interface such that both claimed and unclaimed messages will
be included in the results. A new kwarg was added to the method
so that the caller can override the default behavior.

Change-Id: I21309e986e3e0a501ca14dbfec72321fd6279565
Fixes: bug #1202430
Merge "Add support for listing claimed messages (storage)"