…message

A gochannel subscriber could prevent subscribers which were registered
afterwards from receiving a message.

Steps to reproduce:
1. Create a subscriber A subscribing to topic T.
2. Create a subscriber B subscribing to topic T.
3. Send a message on topic T.
4. Subscriber B will never receive a message as long as subscriber A
   doesn't call ack.

This pull request fixes this behaviour by distributing the messages
concurrently.

Fixes ThreeDotsLabs#247.