I found that deleting an xunit from the database was very, very slow.
This patch addresses the issue by adding additional indexes on the
testcases table to make it easier to find messages that are still in
use. I also rework the `cleanup_testcase_messages` trigger to only check
messageids that were used by the deleted testcase.
Signed-off-by: Anna Schumaker <anna@nowheycreamery.com>