Fixed #19861 -- Transaction ._dirty flag improvement

There were a couple of errors in ._dirty flag handling:
  * It started as None, but was never reset to None.
  * The _dirty flag was sometimes used to indicate if the connection
    was inside transaction management, but this was not done
    consistently. This also meant the flag had three separate values.
  * The None value had a special meaning, causing for example inability
    to commit() on new connection unless enter/leave tx management was
    done.
  * The _dirty was tracking "connection in transaction" state, but only
    in managed transactions.
  * Some tests never reset the transaction state of the used connection.
  * And some additional less important changes.

This commit has some potential for regressions, but as the above list
shows, the current situation isn't perfect either.

docs/ref/models/querysets.txt

@@ -1249,11 +1249,6 @@ make the call non-blocking. If a conflicting lock is already acquired by
 another transaction, :exc:`~django.db.DatabaseError` will be raised when the
 queryset is evaluated.
 
-Note that using ``select_for_update()`` will cause the current transaction to be
-considered dirty, if under transaction management. This is to ensure that
-Django issues a ``COMMIT`` or ``ROLLBACK``, releasing any locks held by the
-``SELECT FOR UPDATE``.
-
 Currently, the ``postgresql_psycopg2``, ``oracle``, and ``mysql`` database
 backends support ``select_for_update()``. However, MySQL has no support for the
 ``nowait`` argument. Obviously, users of external third-party backends should