Changed database connection duplication technique.

This new technique is more straightforward and compatible with test
parallelization, where the effective database connection settings no
longer match settings.DATABASES.

django/db/backends/base/base.py

@@ -1,3 +1,4 @@
+import copy
 import time
 import warnings
 from collections import deque
@@ -622,3 +623,16 @@ class BaseDatabaseWrapper(object):
                 func()
         finally:
             self.run_on_commit = []
+
+    def copy(self, alias=None, allow_thread_sharing=None):
+        """
+        Return a copy of this connection.
+
+        For tests that require two connections to the same database.
+        """
+        settings_dict = copy.deepcopy(self.settings_dict)
+        if alias is None:
+            alias = self.alias
+        if allow_thread_sharing is None:
+            allow_thread_sharing = self.allow_thread_sharing
+        return type(self)(settings_dict, alias, allow_thread_sharing)