Call pgstat_drop_database during DROP DATABASE, so that any stats file

entries for the victim database go away sooner rather than later.  We already
did the equivalent thing at the per-relation level, not sure why it's not
been done for whole databases.  With this change, pgstat_vacuum_tabstat
should usually not find anything to do; though we still need it as a backstop
in case DROPDB or TABPURGE messages get lost under load.