autovacuum: automatically propagate updated parameters

From the documentation:

Parallel workers launched for Parallel Vacuum are using the same cost
delay parameters as the leader worker. If any of these parameters are
changed in the leader worker, it will propagate the new parameter
values to all of its parallel workers.

But in practice, parallel_vacuum_propagate_shared_delay_params was
only called during config reload. Otherwise when the leader adjusted
the parameters, it didn't share them with the other workers.

See the attached patch which adds a test case about this and adds an
additional parallel_vacuum_propagate_shared_delay_params call to the
update logic.