- The before_flush() hook on SessionExtension takes place

- The before_flush() hook on SessionExtension takes place
before the list of new/dirty/deleted is calculated for the
final time, allowing routines within before_flush() to
further change the state of the Session before the flush
proceeds.   [ticket:1128]

- Reentrant calls to flush() raise an error.  This also
serves as a rudimentary, but not foolproof, check against
concurrent calls to Session.flush().