Remove io prefix from pg_stat_io columns

Remove io prefix from pg_stat_io columns

a9c70b46 added the statistics view pg_stat_io which contained columns
"io_context" and "io_object".  Given that the columns are in the
pg_stat_io view, the "io" prefix is somewhat redundant, so remove it.

The code variables referring to these fields are kept unchanged so as
they can keep their context about I/O.

Bump catalog version.

Author: Melanie Plageman
Reviewed-by: Kyotaro Horiguchi, Fabrízio de Royes Mello
Discussion: https://postgr.es/m/CAAKRu_aAQoJWrvT2BYYQvJChFKra_O-5ra3jhzKJZqWsTR1CPQ@mail.gmail.com