Start new workers as more states are killed

[ggrieco-tob]

Fix for #2427

[ehennenfent]

At the meeting, we discussed how checking the number of killed states across each iteration of this loop might be a little bit brittle in a multithreaded context. Fortunately, when a worker dies, Manticore emits an event called did_terminate_worker. Instead of having to check the number of killed states, you could instead create did_terminate_worker_callback on ManticoreBase and then add self.subscribe("did_terminate_worker", self.did_terminate_worker_callback) to __init__. That would simplify the code and reduce the risk of race conditions.

The other thing to note is that we may still want some sort of a bail-out mechanism in case things have gone so wrong that all the workers are dying as soon as we re-create them. Perhaps we could have a maximum number of restarts allowed for each worker ID? Or maybe a maximum number of restarts in five-minute window?

[ggrieco-tob]

@ehennenfent I reimplemented this PR using did_terminate_worker_callback, but I still need to count the number of killed states to know how many of them I need to restart. This is required since did_terminate_worker_callback can be called in a variety of scenarios, where most of them are fine.

On the other hand, some sort of a bail-out mechanism if we restarted too many workers, but keep in mind that manticore should eventually finished because killed states are not revived, so the analysis will finish and manticore will report to the user that some states are unexplored. Perhaps restarting 10 or 20 workers is ok, if you are exploring 2000 states, so it is difficult to determinate. I'm still open to ideas about it.

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