Currently, the idea of using 3 ZeroMQ's IO threads to send messages from the master to its workers is that 3 workers will be served in parallel, and as soon as one message send is complete then the next one starts.

That may not be true. With an example similar to this:

common = runif(1e8) # 700 Mb
clustermq::Q(mean, i=1:100, const=list(x=common), log_worker=T)
# check the logs for call waits, they should be staggered

we observed all workers processing their first call with the almost identical delay (wait in logs).

We should never send common data to all workers in parallel, as this will saturate a slow network interface in case of using many workers, and not leave enough bandwidth for other communication.

Related: #213