Fix a blocking subscriber prevents other subscribers from receiving a message

[boreq]

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 #247.

[roblaszczak]

LGTM, thanks for the contribution 🍻

[skast96]

Is this already in the newest version of watermill ? I am experiencing the same error, except that my subscriber A is "dead" and I need a way to unsubscribe it.

[zhjdhx]

这是来自QQ邮箱的假期自动回复邮件。 您好，我现在有事，无法亲自回复您的邮件。我将在看到邮件的第一时间给您回复。