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 <melanieplageman@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CALdSSPhAU56g1gGVT0%2BwG8RrSWE6qW8TOfNJS1HNAWX6wPgbFA%40mail.gmail.com