vacuum code.

After commit b61d161c, during vacuum, we cache the information of
relation name and relation namespace in local structure LVRelStats so that
we can use it in an error callback function.  We can use the cached
information to avoid the calls to RelationGetRelationName(),
RelationGetNamespace() and get_namespace_name().  This is mainly for the
consistent in vacuum code path but it will avoid the extra syscache lookup
we do in get_namespace_name().

Author: Justin Pryzby
Reviewed-by: Amit Kapila
Discussion: https://www.postgresql.org/message-id/20191120210600.GC30362@telsasoft.com