make the db session explicit for each worker thread

it's a hack and probably not the best way to do it.
on the other hand, I hope it makes the occasional slqalchemy
errors related to threads go away.

(they already got less by the explicit session.close())
modifications to the request html output

- display the status: None, FAIL, or SUCCESS
- display a list of status descriptions instead of
  only the last one (in a hacky way)