Revert "sessions: Stop using the cached_db sessions engine."

This reverts commit 2fa77d9d54.

Further investigation has determined that this did not fix the
password-reset problem described in the previous commit message;
meanwhile, it causes other problems.  We still need to track down the
root cause of the original password-reset bug.

zerver/lib/sessions.py

@@ -49,7 +49,7 @@ def delete_all_deactivated_user_sessions() -> None:
     for session in Session.objects.all():
         user_profile_id = get_session_user(session)
         if user_profile_id is None:
-            continue  # nocoverage # to debug
+            continue
         user_profile = get_user_profile_by_id(user_profile_id)
         if not user_profile.is_active or user_profile.realm.deactivated:
             logging.info("Deactivating session for deactivated user %s" % (user_profile.email,))

zproject/settings.py

@@ -650,9 +650,7 @@ RABBITMQ_PASSWORD = get_secret("rabbitmq_password")
 # CACHING CONFIGURATION
 ########################################################################
 
-# TODO: Restore this to django.contrib.sessions.backends.cached_db
-# after debugging; see https://github.com/zulip/zulip/issues/10856 for details.
-SESSION_ENGINE = "django.contrib.sessions.backends.db"
+SESSION_ENGINE = "django.contrib.sessions.backends.cached_db"
 
 # Compress large values being stored in memcached; this is important
 # for at least the realm_users cache.