Scan GiST indexes in physical order during VACUUM.

Scan GiST indexes in physical order during VACUUM.

Scanning an index in physical order is faster than walking it in logical
order, because sequential I/O is faster than random I/O. The idea and code
structure is borrowed from B-tree vacuum code.

Patch by Andrey Borodin, with changes by me. Based on early work by
Konstantin Kuznetsov, although the patch has been rewritten multiple times
since his original version.

Discussion: https://www.postgresql.org/message-id/1B9FAC6F-FA19-4A24-8C1B-F4F574844892%40yandex-team.ru