pg_rewind: Replace the hybrid list+array data structure with simplehash.
Now that simplehash can be used in frontend code, let's make use of it.
Reviewed-by: Kyotaro Horiguchi, Soumyadeep Chakraborty
Discussion: https://www.postgresql.org/message-id/
0c5b3783-af52-3ee5-f8fa-
6e794061f70d%40iki.fi