Make vacuum buffer counters 64 bits wide

Make vacuum buffer counters 64 bits wide

Using 32 bit counters means they can now realistically wrap around when
vacuuming extremely large tables.  Because they're signed integers,
stats printed by vacuum look very odd when they do.

We'd love to backpatch this, but refrain because the variables are
exported and could cause third-party code to break.

Reviewed-by: Julien Rouhaud, Tom Lane, Michael Paquier
Discussion: https://postgr.es/m/20200131205926[email protected]