Multiple remote invoker bindings for ClientUserTransaction

Remote EJB users might want to enable different invoker bindings
for their EJBs so that they can be accessed via different invokers.

However currently, UserTransaction can only be bound with one
invoker at the time.

This feature request is to enable binding UserTransaction
with multiple invokers so that remote clients can locate the
UserTransaction that best suits their invoker requirements.

Notes:

ClientUserTransactionService currently binds to java:/UserTransaction
and this is not configurable (comes from a static variable). There's
another binding as well related to the UserTransaction factory, whose
lookup is currently hardcoded as it's expected there's only
ClientUserTransactionService with a single proxy/transport type.