Accidental data sending via closures

[mschubert]

Users may inadvertently send hundreds of gigabytes of data via the network by using closures. Consider the following:

do_my_stuff = function() {
    huge_object = runif(1e9)
    my_parallel = function(i) {
    }
    Q(my_parallel, i=1:1000, n_jobs=1000)
}

This will send 7.6 Gb to 1000 workers (= 7.6 Tb total), without any of the workers requiring it.

It should not be that easy to make this mistake.

Related: #200, likely re-introduced with globals package changes close to 9f01bbc