Allow blocking when producing with the async producer

[dasch]

Currently, when calling #produce on an async producer when the message queue is full a BufferOverflow exception is raised. It would be nice to have the option of blocking instead. Bonus points if there's a configurable timeout.

Note that in order to do this correctly a more sophisticated bounded queue would need to be implemented.

[hkroger]

This would be nice since it's also impossible to check the queue length at the moment to halt producing messages if queue is getting too long.

[dasch]

Closing due to a lack of activity.