Add setting CELERY_HAYSTACK_IGNORE_RESULT

[Natureshadow]

If True, results of the signal handler task will not be written to any result store to not produce large amounts of irrelevant results.

Errors will still be stored.

[Natureshadow]

@bigjust Any opinion :)?

[Natureshadow]

This feature is now implemented in our fork: https://pypi.org/project/celery-haystack-ng/