Fix md5_password_warnings for role and database settings

Fix md5_password_warnings for role and database settings

MD5 authentication warnings are queued during authentication, before
startup options and role/database settings have been applied. The code
checked md5_password_warnings at queue time, so settings such as
ALTER ROLE ... SET md5_password_warnings = off did not suppress the
warning, even though the established session showed the GUC as off.

Keep the connection-warning infrastructure generic by allowing each
queued warning to carry an optional filter callback. Evaluate that
callback when warnings are emitted, after startup options and
role/database settings have been processed.

Use this for MD5 authentication warnings, while leaving password
expiration warnings unchanged. Add test coverage for an MD5-authenticated
role with md5_password_warnings disabled.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/AE46E42D-5966-4D76-9E64-95EAB01B9FB5@gmail.com