REPACK CONCURRENTLY: fix processing of toasted tuples

REPACK CONCURRENTLY: fix processing of toasted tuples

In order to process tuples inserted or updated while REPACK executes, we
write those tuples to disk and later restore them; however, some forms
of toasted tuples were not being processed correctly.  Fix that.

Also expand the tests a bit for better coverage.

Author: Satya Narlapuram <satyanarlapuram@gmail.com>
Author: Antonin Houska <ah@cybertec.at>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAHg+QDeXb9HM2VGKXQedyCp52GzajJK5KOUdNi6oLjsS0nerQw@mail.gmail.com