Move main-database-specific behaviours to schema
Allow multiple schemas for multiple connections

To add a schema for a new connection, name it «connection_name».schema.xml.
These will be consolidated exactly the way to normal schema.xml will.

In the db_config.yml, add

  additional_datasources:
    «connection_name»:
      adapter:  «adapter»
      connection:
        user:       «…»
        password:   «…»
        dsn:        «…»