Sometimes we would like to create a base image without any DB access, this
assists in creating custom base images with custom plugins that already
includes `public/assets`
Following this change set you can run:
```
SPROCKETS_CONCURRENT=1 DONT_PRECOMPILE_CSS=1 SKIP_DB_AND_REDIS=1 RAILS_ENV=production bin/rake assets:precompile
```
Then it is straight forward to create a base image without needing a DB or
Redis.
This backend is a bit faster and well tested, this is part of a longer
term plan to have a `backend: :memory, threaded: false` type config for
message bus which we can use in test.
The threading in message bus causes all sorts of surprises in test, it will
be nice not to be beholden to them.
Adds `DISCOURSE_MESSAGE_BUS_REDIS_ENABLED` env var, that when set
to true, will allow Discourse to connect to a different redis
instance for MessageBus needs.
When enabled you can configure the same env vars user for redis,
but prefixed by `MESSAGE_BUS`, eg:
`DISCOURSE_MESSAGE_BUS_REDIS_HOST`
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
