Merge: gamnit::network: add server discovery via UDP

Merge: gamnit::network: add server discovery via UDP

Move server discovery logic from Tinks! up to `gamnit::network` and extend existing services with more options. These new services allow for an easy discovery of local servers over UDP, as in the following example which also creates a server if none is found:

~~~nit
import gamnit::network

# Discover local servers
var servers = discover_local_servers
if servers.not_empty then
	# Try to connect to the first local server
	var server_info = servers.first
	var server = new RemoteServer(server_info)

	if not server.connect then
		print_error "Failed to connect to {server_info.address}:{server_info.port}"
	else if not server.handshake then
		print_error "Failed handshake with {server_info.address}:{server_info.port}"
	else
		# Connected!
		print "Connected to {server_info.address}:{server_info.port}"

		# Write something and close connection
		server.writer.serialize "hello server"
		server.socket.as(not null).close
	end
else
	# Create a local server
	var connect_port = 38271
	print "Launching server: connect on {connect_port}, discovery on {discovery_port}"
	var server = new Server(connect_port)

	# Don't loop if testing
	if "NIT_TESTING".environ == "true" then exit 0

	loop
		# Respond to discovery requests
		server.answer_discovery_requests

		# Accept new clients
		var new_clients = server.accept_clients
		for client in new_clients do
			# Read something and close connection
			assert client.reader.deserialize == "hello server"
			client.socket.close
		end
	end
end
~~~

Pull-Request: #2470
Reviewed-by: Jean-Christophe Beaupré <jcbrinfo.public@gmail.com>