Broker doesn't support wildcards

Subscribing to wildcard topics works fine, but the nmqtt broker doesn't support these.

To reproduce try to subscribe to test/+ with the nmqtt broker and send a message to test/hello, nothing happens. Try again with e.g. mosquitto as the broker and the message is delivered just fine.

Start by reproducing the issue with the nmqtt broker: subscribe to test/+ and publish to test/hello, then compare the result with mosquitto. Trace the broker's topic subscription and message-delivery handling; done means wildcard subscriptions deliver matching messages as MQTT clients expect.