Autovacuum loose end mop-up. Provide autovacuum-specific vacuum cost

Autovacuum loose end mop-up.  Provide autovacuum-specific vacuum cost
delay and limit, both as global GUCs and as table-specific entries in
pg_autovacuum.  stats_reset_on_server_start is now OFF by default,
but a reset is forced if we did WAL replay.  XID-wrap vacuums do not
ANALYZE, but do FREEZE if it's a template database.  Alvaro Herrera