Rails won't even attempt to write to a replica. Without this option,
a write attempt would actually run against Postgres, and it would
be up to Postgres to throw a readonly error.
Additionally, marking it as a replica teaches ActiveRecord that it
should not attempt to run migrations against this connection.
https://api.rubyonrails.org/classes/ActiveRecord/DatabaseConfigurations.html#method-i-configs_for
I'm actually pretty sure that the lack of `replica: true`
is why there's currently a db/slave1_schema.rb and db/slave2_schema.rb
f472a6154e