Give WITH-TRANSACTION a convenience return-value

[tdrhq]

Simple stuff, just a little bit of added convenience.

[kmcgivney]

I don't understand the difference between this approach and the prog2 that's there now. prog2 returns the second form right?

I think what we also need here is to allow the user to return multiple values. prog2 can't return multiple values.

Awkward though it might look the way to do it may be

(progn
  (database-start-transaction ,db)
  (multiple-value-prog1
    (progn
      ,@body)
    (mark-transaction-committed ,db)))