There are two kinds of callbacks, commit actions and close actions.
It is assumed that the use of these callbacks is infrequent.  As a
result, the implementation used tuples defined as class attributes to
avoid creation of instance variables to hold callbacks in the common
case.  This implementation is complicated because tuples are
immutable.  Unfortunately, the instance variables were actually
created anyway in tpc_abort() and tpc_vote().

This implementation changes the class attributes __onCloseCallbacks
and __onCommitCallbacks to default to None.  If a callback is
registered with an instance, a list is bound to an instance attribute
of the same name.  When the transaction commits or aborts, the
instance attribute is deleted.

tpc_vote(): Remove the default arguments since they are unused in the
body of the method.