Assert reset agent always set correctly and is active

Assert reset agent always set correctly and is active

Fixed fairly critical issue where the DBAPI connection could be returned to
the connection pool while still in an un-rolled-back state. The reset agent
responsible for rolling back the connection could be corrupted in the case
that the transaction was "closed" without being rolled back or committed,
which can occur in some scenarios when using ORM sessions and emitting
.close() in a certain pattern involving savepoints.   The fix ensures that
the reset agent is always active.

Note for 1.3 the cherry-pick modifies the approach from master as
transaction handling has diverged.

Fixes: #5326
Change-Id: If056870ea70a2d9a1749768988d5e023f3061b31
(cherry picked from commit 4d161ac9c28986e1e022dfb93785767f28d6bfc8)