Extract parallel execution into a dedicated class

[theofidry]

A design issue with the current Parallelization trait is that:

• it relies on the Container for which the behaviour changed
• it provides little way to by-pass the Container which is a problem in a non Symfony App context
• it provides private code but it is actually not protected due to being in a trait
• a lot of verbose code has been added to accommodate the diversity of setups, which is reflected in the number of parameters passed
• the piece of code is a nightmare to test
• in the future we also want to provide a command instead of the trait. The trait can stay as it is handy and extending commands is sometimes awkward/impossible, but the UX could be made better via a command. However the current trait implementation does not play well with it.

I'm proposing to extract it into a dedicated class. The current implementation simply ported all the code as-is. I do plan to rework it to make easier to use.