`with_connection`, `with_hostname` and `establish_connection` no longer
silently fallback to the default db. When we want that fallback
behavior, we have to specifically opt-in.
For example:
- Most hostname-based lookups in Discourse should fallback to the
default site, so that we match the existing behavior for any sites which
may have misconfigured hostnames
- Loops like CachedCounting, keep_readonly_mode and sidekiq-pausable
should defensively skip attempts to connect to sites which are now
missing, so that they don't get blocked by an exception for one missing
site
Depends on https://github.com/discourse/rails_multisite/pull/40