Stream mutations into mainloop to avoid pre-generation delay

[mbj]

Normally the pre-generation of mutant finishes in < 2seconds. So to reduce complexity it was decided (by me) we kill the streaming infrastructure. This also enables to report the amount of mutations to kill early.

Now @filipvanlaenen has a use case for testing mutant against a 26kloc method, here pre-generation takes too long.

Turning the pre-generation into streaming is easy, as the concurrency primitive assumes a stream already. But we'd loose the ability to report the number of mutations before the mainloop.

This ticket is meant as a reminder to explore the situation.