Replace reset_agent with direct call from connection

Replace reset_agent with direct call from connection

Fixed a regression where the "reset agent" of the connection pool wasn't
really being utilized by the :class:`_engine.Connection` when it were
closed, and also leading to a double-rollback scenario that was somewhat
wasteful.   The newer architecture of the engine has been updated so that
the connection pool "reset-on-return" logic will be skipped when the
:class:`_engine.Connection` explicitly closes out the transaction before
returning the pool to the connection.

Fixes: #6004
Change-Id: I5d2ac16cac71aa45a00b4b7481d7268bd828a168