Fix support for autofields in non-default databases

[untitaker]

Fix #783

Also refactored code a bit such that version detection is no longer used
to determine whether BigAutoField exists. BigAutoField exists in Django
versions older than 3.0, and the way this is currently monkeypatched
prevents us from using any BigAutoField in Spanner with Django 2.2.

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[untitaker]

Turns out, neither commit in this PR is sufficient. The django migration recorder can run in any DB, so router.db_for_write is useless.

Need to patch the querybuilder to succeed IMO.

[untitaker]

I am convinced that the validate_autopk_value function is the right place to hook, as it does not require monkeypatching and can be defined per db backend, but that function will not be called unless there is a value for the autofield. Which would bring us back to patching models/fields.