Optionally circumvent ThreadAction in ArjunaCore when using concurrent nested transactions

Needs sanity checking
We can collaborate on the work via linked GitHub branch
The problem out of the box is when you AtomicAction "begin" - you will link to the ThreadActionData, as you can see in the test I can circumvent that via a different API call start passing in the parent directly.
You can see there is precedent to manipulate ThreadActionData: ArjunaCore/arjuna/classes/com/arjuna/ats/arjuna/coordinator/AsyncPrepare.java (doing the prepare in multiple threads)

I think this is feasible, it may need some thread safety via synchronization in the AtomicAction.

Nested Transactions in core look to be able to support concurrent access except:
1. Threading may not work, will need to check and fix
2. ThreadActionData imposes a stack

You can see in my test (linked in the GitHub branch) I am circumventing the stack.

we already support nested transactions, it should just be not associating them with a thread and making sure they are thread safe when they talk to their parents