GDAL raster methods return different error codes from the vector
API.

Refs #23804.

The failure was introduced in dd1ea707.
The tests wouldn't start due to a "Dependency on unknown app" error.

Refs: #15346, #15573, #16353, #16828.

Thanks James Rivett-Carnac for the report and Markus Holtermann
for the review.

It was originally added to fix a test (refs #7514); but Site now has a
pre_save signal handler (refs #19698) to clear the cache which makes
this call redundant.

StringIO import was adapted for compatibility with Python 2.

Creating a new cluster is neither required not recommendable for
most users. The previous section explains how to create a user
with sufficient permissions to automatically create a database
during tests.

Warnings could escape depending on the order in which tests were run.

One handler in WSGIServer, to catch the error when raised from
SocketServer.BaseServer's finish_request, and one in WSGIRequestHandler
(by creating a subclass of ServerHandler), to catch the error when
raised in wsgiref.handlers.BaseHandler's finish_response.

If no changes that need migrations are found, `makemigrations --exit`
exits with error code 1.

Also removed Query.join_map. This structure was used to speed up join
reuse calculation. Initial benchmarking shows that this isn't actually
needed. If there are use cases where the removal has real-world
performance implications, it should be relatively straightforward to
reintroduce it as map {alias: [Join-like objects]}.

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

Thanks to Berker Peksag and Tim Graham for the review and suggestions.

Thanks Jonathan Lindén for the initial patch, and Tim Graham
and Gabe Jackson for the suggestions.