Thanks Anton Gilgur for the report and review.

Area function, Distance function, and distance lookups now work with
geodetic coordinates on SpatiaLite.

* Avoid some unnecessary attribute lookups, e.g. access signals directly rather than from module
* Alias some repeat accesses inside the method to use the slightly faster local lookups
* Use tuple to iterate remaining kwargs as it's faster to construct
* Cache Field.get_default() to avoid running through all the logic on every call
* Use a cached list of the properties on the model class to avoid repeat isinstance() calls

MySQL schema changes must be done in TransactionTestCase.

Fixed a regression in bacdfbf3.

Thanks Carl Meyer for review.

Fixed definition of `klass_info['from_parent']` so that two models aren't
considered from a parent class if the model classes are the same.