Fixed #24743, #24745 -- Optimized migration plan handling

The change partly goes back to the old behavior for forwards migrations
which should reduce the amount of memory consumption (#24745). However,
by the way the current state computation is done (there is no
`state_backwards` on a migration class) this change cannot be applied to
backwards migrations. Hence rolling back migrations still requires the
precomputation and storage of the intermediate migration states.

This improvement also implies that Django does not handle mixed
migration plans anymore. Mixed plans consist of a list of migrations
where some are being applied and others are being unapplied.

Thanks Andrew Godwin, Josh Smeaton and Tim Graham for the review as well
as everybody involved on the ticket that kept me looking into the issue.

tests/migrations/test_executor.py

@@ -1,5 +1,6 @@
 from django.apps.registry import apps as global_apps
 from django.db import connection
+from django.db.migrations.exceptions import InvalidMigrationPlan
 from django.db.migrations.executor import MigrationExecutor
 from django.db.migrations.graph import MigrationGraph
 from django.db.migrations.recorder import MigrationRecorder
@@ -145,6 +146,64 @@ class ExecutorTests(MigrationTestBase):
         executor.recorder.record_unapplied("migrations", "0002_second")
         executor.recorder.record_unapplied("migrations", "0001_initial")
 
+    @override_settings(MIGRATION_MODULES={
+        "migrations": "migrations.test_migrations",
+        "migrations2": "migrations2.test_migrations_2_no_deps",
+    })
+    def test_mixed_plan_not_supported(self):
+        """
+        Although the MigrationExecutor interfaces allows for mixed migration
+        plans (combined forwards and backwards migrations) this is not
+        supported.
+        """
+        # Prepare for mixed plan
+        executor = MigrationExecutor(connection)
+        plan = executor.migration_plan([("migrations", "0002_second")])
+        self.assertEqual(
+            plan,
+            [
+                (executor.loader.graph.nodes["migrations", "0001_initial"], False),
+                (executor.loader.graph.nodes["migrations", "0002_second"], False),
+            ],
+        )
+        executor.migrate(None, plan)
+        # Rebuild the graph to reflect the new DB state
+        executor.loader.build_graph()
+        self.assertIn(('migrations', '0001_initial'), executor.loader.applied_migrations)
+        self.assertIn(('migrations', '0002_second'), executor.loader.applied_migrations)
+        self.assertNotIn(('migrations2', '0001_initial'), executor.loader.applied_migrations)
+
+        # Generate mixed plan
+        plan = executor.migration_plan([
+            ("migrations", None),
+            ("migrations2", "0001_initial"),
+        ])
+        msg = (
+            'Migration plans with both forwards and backwards migrations are '
+            'not supported. Please split your migration process into separate '
+            'plans of only forwards OR backwards migrations.'
+        )
+        with self.assertRaisesMessage(InvalidMigrationPlan, msg) as cm:
+            executor.migrate(None, plan)
+        self.assertEqual(
+            cm.exception.args[1],
+            [
+                (executor.loader.graph.nodes["migrations", "0002_second"], True),
+                (executor.loader.graph.nodes["migrations", "0001_initial"], True),
+                (executor.loader.graph.nodes["migrations2", "0001_initial"], False),
+            ],
+        )
+        # Rebuild the graph to reflect the new DB state
+        executor.loader.build_graph()
+        executor.migrate([
+            ("migrations", None),
+            ("migrations2", None),
+        ])
+        # Are the tables gone?
+        self.assertTableNotExists("migrations_author")
+        self.assertTableNotExists("migrations_book")
+        self.assertTableNotExists("migrations2_otherauthor")
+
     @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations"})
     def test_soft_apply(self):
         """