Have verify_heapam.c treat unlogged relations as if they were simply
empty when in Hot Standby mode.  This brings it in line with
verify_nbtree.c, which has handled unlogged relations in the same way
since bugfix commit 6754fe65.  This was an oversight in commit
866e24d4, which extended contrib/amcheck to check heap relations.

In passing, lower the verbosity used when reporting that a relation has
been skipped like this, from NOTICE to DEBUG1.  This is appropriate
because the skipping behavior is only an implementation detail, needed
to work around the fact that unlogged tables don't have smgr-level
storage for their main fork when in Hot Standby mode.

Affected unlogged relations should be considered "trivially verified",
not skipped over.  They are verified in the same sense that a totally
empty relation can be verified.  This behavior seems least surprising
overall, since unlogged relations on a replica will initially be empty
if and when the replica is promoted and Hot Standby ends.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Reviewed-By: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-Wzk_pukOFY7JmdiFLsrz+Pd3V8OwgC1TH2Vd5BH5ZgK4bA@mail.gmail.com
Backpatch: 14-, where heapam verification was introduced.