Actively unset reset agent in discard transaction

Actively unset reset agent in discard transaction

The assumptions in _discard_transaction from
916e1fea25afcd07fa1d1d2f72043b372cd02223 were too narrow,
assuming that if the given transaction were not our
"current" one, that this would not be the reset agent.  however
as the legacy behvaior is that even a "nested" transaction gets
set as "self._transaction", this did not accommodate for the nested
transaction being thrown away.   We will attempt to refine all of this
logic in #5327 for 1.4 /master assuming this is feasible for the
full suite of current use cases.

Fixes: #5326
Change-Id: I6787e82c9e50c23317f87d0d094122c6a6f066da