python - fe: Use Transactor attribute for Function execution.

Log Message:
-----------
Use Transactor attribute for Function execution.

This gives clients the ability to specify what client3.Transaction to use to
execute the StoreProcedure. (aids the remote console project)

Additional improvements and changes:

In Transact(), only rollback if the connection is not closed.
In Interchange.__call__(), use the reader() and writer() locals when
synchronizing after an error.

Modified Files:
--------------
    fe/src:
        tracenull.py (r1.21 -> r1.22)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/fe/src/tracenull.py.diff?r1=1.21&r2=1.22)