Stop use of boolean enum pattern in SpreadGenerator

An actual enum that wraps the behaviour is better. The code also
explains itself better like this.

It would be nice to have a better way to handle the selection of the
AtomicLong.
Add class to collect the filtered seeds
Delete collector

Just mapToObj instead.
Add a tracker to generate, filter, and collect the seeds
Add contention tests for spread generator

They expose some problems with the generation of numbers!
Fix the tests to correctly request the generator values