Update data model to store messages in reverse order on disk

[rstml]

From: http://www.datastax.com/dev/blog/advanced-time-series-with-cassandra

Since Cassandra 0.8, column comparators can easily be reversed. This means that if you’re using timestamps or TimeUUIDs as column names, you can choose to have them sorted in reverse chronological order.

If the majority of your queries ask for the N most recent events in a timeline or N events immediately before a point in time, using a reversed comparator will give you a small performance boost over always setting reversed=True when fetching row slices from the timeline.

This should be useful for MessageMetadata and IndexLabels CFs