Refactor of trytond.config to add cache

This removes the need to store configuration in global variable and so make it
easier to update for example for testing.

Closes #14249

modules/inbound_email/inbound_email.py

@@ -11,7 +11,7 @@ from email.policy import default as email_policy_default
 from email.utils import getaddresses
 from functools import partial
 
-from trytond.config import config
+import trytond.config as config
 from trytond.model import (
     ModelSQL, ModelStorage, ModelView, fields, sequence_ordered)
 from trytond.pool import Pool

modules/inbound_email/routes.py

@@ -3,7 +3,7 @@
 
 import json
 
-from trytond.config import config
+import trytond.config as config
 from trytond.protocols.wrappers import (
     HTTPStatus, Response, abort, set_max_request_size, with_pool,
     with_transaction)