Use main database terminology

In a team meeting (internal), we had a discussion about the main database in contrast to primary / replica.

In a informal Slack poll (internal), we in groupsharding settled on main.

• Update docs
• Make our own wrapper helper so that we hide rails' ActiveRecord::Base.configurations.primary? method. Also it's a ::nodoc:: method, so I will feel a lot better if we have some tests against this so we know if/when Rails breaks this.