Sealed interfaces are useful in switch exprs to enumerate possible values
so that it becomes a compile error to introduce a new server type
without implementing an appropriate arm for the switch block.

ServiceAnnouncer requires a non-null String, but an empty server ID is
not well-formed so a ServiceAnnouncer shouldn't be created.

This was previously handled by MapToolServer being passed null config
and the absence of a port to give was sufficient to prevent creating a
ServiceAnnouncer with an empty ID,
but we can be more defensive about this and check for a null pointer.

This adds extra complexity from the ServiceAnnouncer not always existing
but enables later flexibility in supporting binding to an emphemeral
port instead of having to preconfigure it.

It is neater to stop announcement, deregister and close UPnP
before stopping the server so that discovering clients
can't try to connect to an already stopped server
though this is mostly just being picky because the human interaction
time scales dominate and shut down and deregister are nearly the same
time.

If a port number of 0 is chosen then bind() will open a random port in
an ephemeral range instead of the provided port number
and this port can then be retrieved using getLocalPort().

This enables starting servers on arbitrary ports instead of
preconfigured ones.

Since only direct connections need to know the port number
connections using the registry or LAN can use an ephemeral port number
and the connection information tab can display the port direct
connections should use.

This requires a router with functioning UPnP instead of port forwarding
though so is not univerally useful.