zb: Bigger broadcast channel for PropertyStream

If there are multiple property updates in a PropertiesChanged signal, we could override the updates before all streams have received them. A bigger channel ensures that it at least doesn't happen easily.

TODO: It's still a leaky channel as we enable the overflow mode so this patch doesn't fully address the problem. I tried to disable the overflow mode but the issue there is that broadcast method will wait not only in case of a full channel but also if there are no active receivers (i-e no PropertyStream instances). So that part requires some thinking (likely needing a task).