Read toast chunks in ascending order, looping if required

Read toast chunks in ascending order, looping if required

Toast chunks might be stored out-of-order in the table. We previously
ignored that and hoped for the best, but our toast.sql test actually
exhibits the problem on pre-14 servers where the 5th chunk of the last
test is small enough to fit into the first disk block.

Fix by looping over the table until all chunks have been read. A smarter
solution would require either toast index lookups or caching the chunks.

Close #20.

Author: Christoph Berg <myon@debian.org>
Debugging-By: Svetlana Derevyanko <s.derevyanko@postgrespro.ru>