proxyserver: add support for UnifiedPush standard

Make the proxy able to support the UnifiedPush platform, so that
applications using the proxy server can use any UnifiedPush provider.

Documentation: https://unifiedpush.org/

How to test:
+ Install a UnifiedPush provider on a device (e.g. https://ntfy.sh)
+ Run dhtnode with a proxyserver and --unifiedpush https://ntfy.sh
+ Subscribe to a random topic on ntfy (e.g. "dhtproxytest")
+ Subscribe to a hash on the DHT:
    curl -X SUBSCRIBE http://my.dht.proxy:8000/key/foo \
         -d '{"key":"dhtproxytest", "platform":"unifiedpush"}'
+ Send data on the subscribed hash

Note: the topic should be completely random.