* [database/*] Allow databases other than Redis to provide pubsub for clustering if Redis is not present
* [pubsub] Delay messages sent before the database is ready until the database is ready.
* [pubsub] Restore old behavior of not using the database in non-clustered NodeBB instances.
See comment: https://github.com/NodeBB/NodeBB/pull/6233#issuecomment-357814968