Simplify get_serialized_models via queryset slicing
Don't save counters_* to self, as it suggests we're saving state in the session--in fact, we clear them explicitly, so they should be stateless local variables
Merge branch 'counters_at_sync' of github.com:bcipolli/ka-lite into issue631
Merge branch 'counters_at_sync' of github.com:bcipolli/ka-lite into issue631
Fixed two bugs (both previously staged, but misplaced during a branch change).
Adding two settings to allow throttling of syncing data, to reduce CPU load.  Did not add anything under CONFIG_PACKAGE="RPi"
Merge branch 'develop' of github.com:learningequality/ka-lite into issue631

Conflicts:
	kalite/securesync/engine/api_client.py
Merge pull request #757 from bcipolli/issue631

Securesync: Updating logic for collecting models to send over the wire (#631)