Refactor Continuous Aggregate catalog code

Get rid of `GETSTRUCT` to fill the form data and use `heap_deform_tuple`
instead. This is necessary specially when you have variable lenght
fields and/or fields that accept NULL values. This refactoring will be
specially usefull in a following PR for Nested Continuous Aggregates
where we'll add a new metadata to the catalog that can accept NULL
values.

Also refactor the rename view and schema code paths improving the
readability and maintainability.