Refactor Rex::TaskList. No functional change.

[kablamo]

I'm trying to clean up the code to make way for new functionality. This pr is purely a refactor and there should be no functional changes introduced. Summary of the changes:

• Moved $forked_sub out of Rex::TaskList->run() and into its own function.
• This function is now shared between Rex::TaskList::Base and Rex::TaskList::Parallel_ForkManager.
• Removed functions from Rex::TaskList::Parallel_Forkmanager which are already in Rex::TaskList::Base.

[kablamo]

Ready for review

[ferki]

It's a dependency of #851/#855.

[krimdomu]

Hi @kablamo ,

this is now merged into master.