Skip WAL for unlogged main fork during online checksum enable

Skip WAL for unlogged main fork during online checksum enable

ProcessSingleRelationFork() unconditionally generated an FPI WAL
record for every page of every relation when enabling checksums.
Unlogged relations, which by definition never generate WAL for
data changes, were not exempt which generated excessive WAL to
be emitted.

Fix by guarding the FPI WAL record call with RelationNeedsWAL()
to avoid emitting WAL for unlogged main forks.  Unlogged pages
are still dirtied to ensure the checksum is written to disk at
the next checkpoint.  The init fork remains WAL-logged even for
unlogged relations, as it's needed on the standby to materialize
the relation after promotion (see ResetUnloggedRelations()).
Skipping init-fork WAL would leave the standby with a stale init
fork that, once copied to the main fork on promotion, would fail
checksum verification on every read of the unlogged relation.

A test which creates an unlogged table with an index, enables
checksums, promotes the standby, and verifies that the unlogged
relation and its indexes are still readable post-promotion has
been added.

Author: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Discussion: https://postgr.es/m/CAHg+QDeGrpZbNZdLjd_T4b43xKEEXZN0HGhkFm-1bkBdyzK7AQ@mail.gmail.com