Comment 3 for bug 879589

This requires some design level thought: we need to:
  - preserve the audit facility
  - including the ability to incrementally audit bugs (which means that
 new observers and new subscribers need to bring the bug forward in
 searches) - observer grants already have a datestamp to permit this
  - meet other sorts of incremental operations like 'bug status changed
 since X' and/or 'bug status or comments changed since X and/or 'bug
 status or comments or tasks changed since X'.

 Essentially, a bit of a rethink on the date fields we have, when they
 change, and how, and then tie it all up in a bow and migrate things
 across. last-changed is essentially a denormalisation, and as such
 there are many different summaries we might use - so we may need to
 e.g. have several fields, or we may need to improve bugactivity to permit efficient queries on different sorts of things (tasks changes, comments, subscriptions, private status). Performance testing will be needed as well (obviously) to decide whats best.