add memory clean up on gochannel subscribe topics when they no longer have any more subscribers

[dkotik]

This allows publishing and subscribing to narrowly focused topics that include random data like UUIDs in topic name. Without this change, memory usage will grow indefinitely in a long-running service as the map grows larger and larger with keys pointing to empty slices.

The change is small, but it allows using Watermill Gochannel like NATS - having a topic per HTTP request, for example.