Teach verify_heapam() to validate update chains within a page.

Teach verify_heapam() to validate update chains within a page.

Prior to this commit, we only consider each tuple or line pointer
on the page in isolation, but now we can do some validation of a line
pointer against its successor. For example, a redirect line pointer
shouldn't point to another redirect line pointer, and if a tuple
is HOT-updated, the result should be a heap-only tuple.

Himanshu Upadhyaya and Robert Haas, reviewed by Aleksander Alekseev,
Andres Freund, and Peter Geoghegan.