ts/udpsink: fix default clients not leading to socket configuration

During MR !793 (merged), the socket configuration mechanism was changed to use commands passed to the Task via a channel. This worked properly for user changes via settings and signals, however the default clients setting was not used.

A simple solution could have been to send a command at initialization to add the default clients, but it was considered a better solution to just wait for the Task preparation to configure the sockets based on the value of settings.clients at that time, thus avoiding unnecessary successive removals and additions of clients which could have happened before preparation.

Of course, users can still add or remove clients as before, before and after Task preparation.

See also !793 (merged)