Gitlab::Database has both read_only? and db_read_only? that do different things

The following discussion from gitlab-ce!20345 should be addressed:

• @stanhu started a discussion: (+4 comments)

  Hmm, there is a self.read_only? already defined above?

  I think it's still potentially confusing that a module called Database has both read_only? and db_read_only? methods that don't actually do the same thing.