Restrict pg_stat_io entries for data checksum processes

Restrict pg_stat_io entries for data checksum processes

The data checksums launcher and workers were exposed in pg_stat_io
with the same broad set of object/context combinations as general
background workers. However, several of those entries can never
accumulate I/O statistics for these processes, such as bulkwrite,
relation init, temporary relation, and launcher vacuum entries.

Teach pgstat_tracks_io_object() and pgstat_tracks_io_op() about the
actual I/O performed by the data checksum processes. Keep the entries
needed for catalog scans, including bulkread catalog scans, worker
relation processing with a vacuum access strategy, and WAL writes and
initialization, while excluding WAL reads and other object/context
combinations that can never be used.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAHGQGwHz_-nt+YkHDMRZNBZrnoHro8cMOgSwuXEmSYT6vxgQ=w@mail.gmail.com
Backpatch-through: 19