Start new workers as more states are killed

[gustavo-grieco]

Fix for #2427

[gustavo-grieco]

@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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