add SmallAutoField support

Django 3.0 adds support for [SmallAutoField](https://github.com/django/django/commit/194d1dfc186cc8d2b35dabf64f3ed38b757cbd98), however, cockroachdb's `unique_rowid()` generates 8-byte values that are too large for `int2` (2-byte). I'm not sure how to proceed.

Test failure:
```
======================================================================
ERROR: test_fk_to_smallautofield (many_to_one.tests.ManyToOneTests)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/tim/code/django/django/db/backends/utils.py", line 86, in _execute
return self.cursor.execute(sql, params)
psycopg2.errors.NumericValueOutOfRange: integer out of range for type int2 (column "country_id")

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/home/tim/code/django/tests/many_to_one/tests.py", line 583, in test_fk_to_smallautofield
City.objects.create(country=us, name='Chicago')
File "/home/tim/code/django/django/db/models/manager.py", line 82, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/home/tim/code/django/django/db/models/query.py", line 433, in create
obj.save(force_insert=True, using=self.db)
File "/home/tim/code/django/django/db/models/base.py", line 746, in save
force_update=force_update, update_fields=update_fields)
File "/home/tim/code/django/django/db/models/base.py", line 784, in save_base
force_update, using, update_fields,
File "/home/tim/code/django/django/db/models/base.py", line 886, in _save_table
results = self._do_insert(cls._base_manager, using, fields, returning_fields, raw)
File "/home/tim/code/django/django/db/models/base.py", line 925, in _do_insert
using=using, raw=raw,
File "/home/tim/code/django/django/db/models/manager.py", line 82, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/home/tim/code/django/django/db/models/query.py", line 1204, in _insert
return query.get_compiler(using=using).execute_sql(returning_fields)
File "/home/tim/code/django/django/db/models/sql/compiler.py", line 1344, in execute_sql
cursor.execute(sql, params)
File "/home/tim/code/django/django/db/backends/utils.py", line 68, in execute
return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
File "/home/tim/code/django/django/db/backends/utils.py", line 77, in _execute_with_wrappers
return executor(sql, params, many, context)
File "/home/tim/code/django/django/db/backends/utils.py", line 86, in _execute
return self.cursor.execute(sql, params)
File "/home/tim/code/django/django/db/utils.py", line 90, in __exit__
raise dj_exc_value.with_traceback(traceback) from exc_value
File "/home/tim/code/django/django/db/backends/utils.py", line 86, in _execute
return self.cursor.execute(sql, params)
django.db.utils.DataError: integer out of range for type int2 (column "country_id")
```