status dates (for workflow and reporting)

in addition to the create/update timestamps, it would be great to have the dates that issues were assighed/inprogress/resolved/(built?, see JRA-568)/reopened/closed.

this would allow us to create escalation workflows - i.e. if a blocker is not marked inprogress within a certain timeframe, as well as management reports, of which I can think of several right offhand:

1. time-to-assign (since created)
2. time-to-resolve (since assigned)
3. time-to-test (since built)
4. number of issues that were reopened (you can't currently tell if an issue was ever reopened once it has been resolved)