Committing serves two purposes:
- obviously: it makes any transactional data change persistent
- less obviously: it releases the database transactional snapshot taken by
  any previously issued query, allowing that snapshot and all snapshots
  after it to be also freed (ex: garbage collecting the data records that
  no other transaction can see)
In the current state of the code flow, this does not change much: after
this method returns the code flow should quickly hit a commit anyway,
either in dequeueMessage or before handing control back to timerserver,
thereby ending the Zope-initiated transaction.
But this will not be true in a future version of the code.