We memorize all internal and empty leaf pages in the 1st vacuum stage for
gist indexes.  They are used in the 2nd stage, to delete all the empty
pages.  There was a memory context page_set_context for this purpose, but
we never used it.

Reported-by: Amit Kapila
Author: Dilip Kumar
Reviewed-by: Amit Kapila
Backpatch-through: 12, where it got introduced
Discussion: https://postgr.es/m/CAA4eK1LGr+MN0xHZpJ2dfS8QNQ1a_aROKowZB+MPNep8FVtwAA@mail.gmail.com