Add transactional consistent state changes, i.e. optimistic locking

[alejandromumo]

          comment/shelve: at some point it would be nice to add transactional consistent state changes, i.e. optimistic locking. That means that e.g. when you update a row you do:

update swh_deposit
set status = 'WAITING'
where 
  id = 123
  and status = 'PENDING';

and then check if any rows were updated (i.e. what we already do for records with the version_id field).

Originally posted by @slint in #37 (comment)