Prevent database connection at PostGISOperations init time

[Claude Paroz]

Currently, PostGISOperations does query the database during its init time (to check the spatial version). This should be avoided if possible (see use case in #17656), and should be doable by transforming some variables into properties.

Traceback (most recent call last):
  File "manage.py", line 12, in <module>
    execute_from_command_line(sys.argv)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
    utility.execute()
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/core/management/__init__.py", line 354, in execute
    django.setup()
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/__init__.py", line 21, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/apps/registry.py", line 108, in populate
    app_config.import_models(all_models)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/apps/config.py", line 197, in import_models
    self.models_module = import_module(models_module_name)
  File "/usr/lib64/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/contrib/auth/models.py", line 40, in <module>
    class Permission(models.Model):
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/models/base.py", line 125, in __new__
    new_class.add_to_class('_meta', Options(meta, **kwargs))
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/models/base.py", line 300, in add_to_class
    value.contribute_to_class(cls, name)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/models/options.py", line 166, in contribute_to_class
    self.db_table = truncate_name(self.db_table, connection.ops.max_name_length())
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/__init__.py", line 40, in __getattr__
    return getattr(connections[DEFAULT_DB_ALIAS], item)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/utils.py", line 243, in __getitem__
    conn = backend.DatabaseWrapper(db, alias)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/contrib/gis/db/backends/postgis/base.py", line 14, in __init__
    self.ops = PostGISOperations(self)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/contrib/gis/db/backends/postgis/operations.py", line 166, in __init__
    if self.spatial_version < (1, 3, 4):
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/utils/functional.py", line 55, in __get__
    res = instance.__dict__[self.func.__name__] = self.func(instance)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/contrib/gis/db/backends/postgis/operations.py", line 260, in spatial_version
    vtup = self.postgis_version_tuple()
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/contrib/gis/db/backends/postgis/operations.py", line 434, in postgis_version_tuple
    version = self.postgis_lib_version()
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/contrib/gis/db/backends/postgis/operations.py", line 414, in postgis_lib_version
    return self._get_postgis_func('postgis_lib_version')
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/contrib/gis/db/backends/postgis/operations.py", line 404, in _get_postgis_func
    with self.connection.temporary_connection() as cursor:
  File "/usr/lib64/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/backends/__init__.py", line 543, in temporary_connection
    cursor = self.cursor()
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/backends/__init__.py", line 165, in cursor
    cursor = self.make_debug_cursor(self._cursor())
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/backends/__init__.py", line 138, in _cursor
    self.ensure_connection()
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/backends/__init__.py", line 133, in ensure_connection
    self.connect()
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/backends/__init__.py", line 133, in ensure_connection
    self.connect()
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/backends/__init__.py", line 122, in connect
    self.connection = self.get_new_connection(conn_params)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/django/db/backends/postgresql_psycopg2/base.py", line 134, in get_new_connection
    return Database.connect(**conn_params)
  File "/home/matt/repos/uwncom/lib/python2.7/site-packages/psycopg2/__init__.py", line 164, in connect
    conn = _connect(dsn, connection_factory=connection_factory, async=async)
django.db.utils.OperationalError: FATAL:  role "234124323" does not exist

[Mattia Procopio]

traceback added