MessageController implementation for mongodb storage

This patch implements MessageController for the mongodb storage. In
order to test the changes made here, it was necessary to modify current
base classes.

This patch also adds more granular exceptions (QueueDoesNotExist and
MessageDoesNotExist) both based on the existing exception DoesNotExist.

Implements blueprint storage-base
Implements blueprint storage-mongodb

Change-Id: I0752c5d797104a9cc58efd8e866b3b627646bce5
Merge "MessageController implementation for mongodb storage"