Originally, uids somehow sorted messages by date of insertion, in particular
for those that were created within the same second. But since random uids,
such messages became validated or processed in random order.

Note however that by default, messages created in the same transaction all have
exactly the same date, so commit a42da4de
("CMFActivity: Do not use offset for scanning messages to validate.")
forces us to keep the ordering on uids (in addition to priority/date).

Existing instances will upgrade automatically, using the already existing code
to upgrade tables in a generic way. You should see the following logs:

  INFO CMFActivity 'message_queue' table upgraded
  ALTER TABLE message_queue
    MODIFY COLUMN date datetime(6) NOT NULL AFTER uid,
    MODIFY COLUMN processing_date datetime(6) DEFAULT NULL AFTER processing
  INFO CMFActivity 'message_job' table upgraded
  ALTER TABLE message_job
    MODIFY COLUMN date datetime(6) NOT NULL AFTER uid,
    MODIFY COLUMN processing_date datetime(6) DEFAULT NULL AFTER processing
  INFO CMFActivity 'message' table upgraded
  ALTER TABLE message
    MODIFY COLUMN date datetime(6) NOT NULL AFTER uid,
    MODIFY COLUMN processing_date datetime(6) DEFAULT NULL AFTER processing