Improve hints for database connections (DatabaseWrapper) (#612)

* `django.db.{connection, connections, router}` are now hinted -- including `ConnectionHandler` and `ConnectionRouter` classes.
* Several improvements to `BaseDatabaseWrapper` attribute hints.
* In many places, database connections were hinted as `Any`, which I changed to `BaseDatabaseWrapper`.
* In a few places I added additional `SQLCompiler` hints.
* Minor tweaks to nearby code.

tests/typecheck/db/test_connection.yml

@@ -0,0 +1,13 @@
+-   case: raw_default_connection
+    main: |
+      from django.db import connection
+      with connection.cursor() as cursor:
+        reveal_type(cursor)  # N: Revealed type is 'django.db.backends.utils.CursorWrapper'
+        cursor.execute("SELECT %s", [123])
+-   case: raw_connections
+    main: |
+      from django.db import connections
+      reveal_type(connections["test"])  # N: Revealed type is 'django.db.backends.base.base.BaseDatabaseWrapper'
+      for connection in connections.all():
+        with connection.cursor() as cursor:
+          reveal_type(cursor)  # N: Revealed type is 'django.db.backends.utils.CursorWrapper'