pgsql: Add assert and log message to visibilitymap_set

Add assert and log message to visibilitymap_set

Add an assert to visibilitymap_set() that the provided heap buffer is
exclusively locked, which is expected.

Also, enhance the debug logging message to specify which VM flags were
set.

Based on a related suggestion by Kirill Reshke on an in-progress
patchset.

Author: Melanie Plageman 
Reviewed-by: Kirill Reshke 
Reviewed-by: Andres Freund 
Discussion: https://postgr.es/m/CALdSSPhAU56g1gGVT0%2BwG8RrSWE6qW8TOfNJS1HNAWX6wPgbFA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e3d5ddb7ca91e5982e9d4cff9eef210d97e4f47e

Modified Files
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src/backend/access/heap/visibilitymap.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)