add index on messages (sent_at) for deleting them

Change-Id: I4ab585b723e9c299eaaa57d3ce3e0cc2ab79bd42
Reviewed-on: https://gerrit.instructure.com/32264
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>