…_PARALLEL default setting and parameters to management command.

Use qs.iterator() for fetching data during reindex, as this is much more memory efficient and performant.
Instead of finding out which methods to call to prepare fields, do that finagling once and cache it for subsequent model instance prepares.

See issue django-es#154 for performance analysis and details.

…ally cleaner. Also shaves off a test per object.

…, as it's conceptually possible to have several indices on the same model.

Also remove forced ordering that is a remnant of earlier code.

… it up as document field

…rysets that might be expensive.

Add tests for usage of init_prepare() and _prepared_fields.
Add tests for correct calling of bulk/parallel_bulk.