Invoke the transaction drivers' .clean() methods only after all
.commit() or .abort() handlers are done.
This makes it easier to have nested transactions where the top-level
transactions pass objects to lower transactions that the latter can
still use throughout their commit/abort phases, while the top-level
transaction keeps a reference that is released in its .clean() method.
(Before this commit, that is also possible, but the top-level
transaction would need to take care to invoke tran_add() before the
lower-level transaction does. This commit makes the ordering
irrelevant, which is just a bit nicer.)
Signed-off-by: Hanna Reitz<hreitz@redhat.com>