WS-AT Subordinat Context Handler needs to reinstate existing subordinate if there is one

The WS-AT subordinate transaction context handler creates a subordinate transaction for an incoming WS-AT transaction and then resumes it, thus ensuring that the web service performs its operations in the subordinate transaction. The problem is that it needs to ensure that on subsequent invocations of the web service in the same top-level transaction it resumes the same subordinate transaction (it currently always creates a subordinate). The hard bit is also ensuring that the parent to subordinate association is garbage collected when either the parent or subordinate commits or rolls back.