UserTransactionSessionImpl is not thread safe

First of all, static TransactionManager check and assignment is not synchronised.

Secondly, WeakValueHashMap activeTx underneath is a HashMap and hence not thread safe.
During load testing, seems like transactions were disappearing from activeTx for the same
reason that transactions were dissapearing from txFailoverAuthorization map which is due to
lack of synchronisation. This map will now become a Collections.synchronizedMap(new WeakValueHashMap());

Finally, in spite of making activeTx map a synchronised map, we still need to synchronise on
activeTx in unreferenced() method as a putIfAbsent type of operation if executed.