Change process model

[mbj]

Some numbers:

Before, on ~20k mutations:

Mutations/s: 10.5

Now, with this change:

Mutations/s: 20.0

These counts are from from a private customer project, with rails "infection".

Some more numbers:

null integration against auom "standard candle":

Before, 990 mutations:

Mutations/s: 250.7

Now:

Mutations/s: 349.5

(and this is a 4s run).

Against minitest subtract ~50 from both.

As usual: The test speet matters more than mutants overhead.

[mbj]

@dgollahon, @pawelpacana This branch passes integration specs but not yet unit/mutation coverage.

Still it should optimize large runs significantly. I even see positive impact on smaller normal runs. But there are also some counter examples on lower core counts.

The key difference is that the main processes memory that grows over time when accumulating the result does NOT get inherited anymore into the individual killforks.

If you have time feel free to give this branch an early trial on your projects.