tamedevil needs identifier tracking improvements

[benjie]

In c16ac7e I broke a naming conflict by prepending a $; but that's not good enough in general. We need tamedevil to track all identifiers and make sure they're always unique. This is the query that caused the issue (running against test database with polymorphic schema):

query Q {
  allRelationalItems(first: 1) {
    nodes {
      id
      type
      relationalItemByParentId {
		id
        type
      }
    }
  }
}

Perhaps we should use symbols like in pg-sql2, where the symbol description is just a hint.