Don't unmap message segments before last consumer is done

[snichme]

Describe the bug

We have a bug (hard to trigger) that we may unmap message segments before the last consumer deliver loop has finish writing to the socket, causing a segmentation fault.

We should rethink when we do the unmap. Maybe a fiber waiting for all deliver loops to be done (using a WaitGroup).

[snichme]

@spuun solved by #850 right?

[spuun]

Yes, #850 solved this.