soft close cursor for Query direct iterator interrupted

soft close cursor for Query direct iterator interrupted

Fixed issue where the underlying DBAPI cursor would not be closed when
using :class:`_orm.Query` and direct iteration, if a user-defined exception
case were raised within the iteration process, interrupting the iterator
which otherwise is not possible to re-use in this context. When using
:meth:`_orm.Query.yield_per` to create server-side cursors, this would lead
to the usual MySQL-related issues with server side cursors out of sync.

To resolve, a catch for ``GeneratorExit`` is applied within the default
iterator, which applies only in those cases where the interpreter is
calling ``.close()`` on the iterator in any case.

A similar scenario can occur when using :term:`2.x` executions with direct
use of :class:`.Result`, in that case the end-user code has access to the
:class:`.Result` itself and should call :meth:`.Result.close` directly.
Version 2.0 will feature context-manager calling patterns to address this
use case.  However within the 1.4 scope, ensured that ``.close()`` methods
are available on all :class:`.Result` implementations including
:class:`.ScalarResult`, :class:`.MappingResult`.

Fixes: #8710
Change-Id: I3166328bfd3900957eb33cbf1061d0495c9df670