Forwardport of ae1d663b
from stable/1.8.x plus more.

Thanks to github aliases dbrgn, carljm, slurms, dfunckt, and timgraham
for reviews.

Complete rework of translating data values from database

Deprecation of SubfieldBase, removal of resolve_columns and
convert_values in favour of a more general converter based approach and
public API Field.from_db_value(). Now works seamlessly with aggregation,
.values() and raw queries.

Thanks to akaariai in particular for extensive advice and inspiration,
also to shaib, manfre and timograham for their reviews.

db_parameters should respect an already existing db_type method and
return that as its type string. In particular, this was causing some
fields from gis to not be generated.

Thanks to @bigsassy and @blueyed for their work on the patch.

Also fixed #22260

Thanks Piotr Kasprzyk for help with the patch.

Signed-off-by: Jason Myers <jason@jasonamyers.com>

The SubfieldBase's descriptor caused an AttributeError when accessed
from the class. Introspection didn't like that.

Patch by Trac alias supervacuo.

Should be unneeded with Python 2.7 and up.
Added some unicode_literals along the way.

Gives much nicer errors when it fails.