fix: make transaction status check account for pipeline mode

fix: make transaction status check account for pipeline mode

We turn _check_intrans() into a generator _check_intrans_gen() in order
to call _pipeline._sync_gen() if the connection is in pipeline mode so
as to retrieve an accurate connection status.

This makes the safety guard about 'autocommit' when inside a transaction
work in pipeline mode, thus removing the xfail in transaction tests.

In test_autocommit_unknown, we now catch OperationalError which is
raised by conn.wait() rather than ProgrammingError previously which is
no longer reached.