Remove some global variables from vacuum.c

Remove some global variables from vacuum.c

Using global variables because we don't want to pass these values around
as parameters does not really seem like a great idea, so let's remove
these two global variables and adjust a few functions to accept these
values as parameters instead.

This is part of a wider patch which intends to allow the size of the
buffer access strategy that vacuum uses to be adjusted.

Author: Melanie Plageman
Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/CAAKRu_b1q_07uquUtAvLqTM%3DW9nzee7QbtzHwA4XdUo7KX_Cnw%40mail.gmail.com