CMFActivity: better date ordering by using micro-precision

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