Support multiple @ServiceRequest calls per transaction, creating a single participant per RM

For example, with WS-BA:

@ServiceRequest
pubic void addToBasket(...)
{
  ...
{

@Completes
public void checkout()
(
  ...
}

@Compensate
public void removeFromBasket(...)
{
  ...
}

Here addToBasket() can be called multiple times within the same transaction. The default behaviour should be to enlist a single participant, rather than one per Service-invocation.

If the transaction is compensated, 'removeFromBasket' is invoked once. A single participant is registered, per transaction, even if many calls to a method annotated with @ServiceRequest occur.

Similar functionality should be provided for WS-AT services.