daklib/ls.py: manage our own database session

daklib/ls.py: manage our own database session

session_wrapper gets confused by generators: let "f" be a generator
making use of a database session:

    def wrapped_f():
      session = DBConn().session()
      try:
        return f(session)
      finally:
        session.close()

    for item in wrapped_f():
      do_something(item)

will leak database sessions as the implementation of "f" will only be
invoked from the for-loop. However at this time the "finally" block
closing the session has already run. SQLAlchemy will helpfully reopen
the session, but it will not be closed in the end.