OTS11 — asynchronous responses

Currently the core of the Additional Structuring Mechanisms for the OTS (aka
Activity Service) assumes synchronous responses from participants (Actions)
at the instigation of the coordinator messages (generated by the SignalSet).
As we have found in several recent studies and other work on extended
transactions (e.g., the OASIS BTP) there are good reasons why participants
may want to asynchronously send "responses" to messages the coordinator
hasn't yet generated. For example, in a two-phase protocol, a participant
may be able to spontaneously prepare and send the coordinator the relevant
Outcome.

In the current spec. this isn't possible directly. Obviously there are
tricks that could be played with another-level-of-indirection (e.g., enlist
a "cacheing" participant with the coordinator that the real Action enlists
with and this receives [and stores] any spontaneous responses). However,
even this doesn't really address the entire problem since the response the
Action sent (the Outcome) is made with respect to a presumed specific Signal
that the coordinator sent. So, what if the coordinator doesn't send that
Signal?

Having this as part of the core will help us to continue to support a wide
range of coordination/extended transaction protocols.