Fix another race condition in threaded monodromy solve

[saschatimme]

Multithreading is hard...

Before, this loop

f = steiner()
for k in 1:50
    monodromy_solve(f, target_solutions_count = 3265, max_loops_no_progress = 2)
end

started at some point to print progress for two computations simultaneously. With these changes I cannot reproduce this anymore.