OTS — SignalSet and participants

There has always been an implicit assumption "something" knows when (and if)
to make references to Actions persistent during SignalSet processing. For
example, if we consider the case of a two-phase commit SignalSet, then once
prepare Signals have been sent and acknowledged successfully by Actions, the
service needs to make those references persistent (c.f. the transaction
service intentions list).

However, currently there is no way for this to happen. Only the Activity
Coordinator has the list of the Actions, but it doesn't (and shouldn't)
understand the semantics of the Signals it sends. The SignalSet understands
the semantics, but doesn't have a handle on the Actions. Somehow the
SignalSet must get hold of the Actions in order to do this.

What I propose is the addition of a set_coordinator method to SignalSet such
that when it is registered with the Activity Coordinator (or more likely
when it is about to be used) the coordinator can pass a reference to itself
through. Then the SignalSet can have access to the Action list. This is also
important if the SignalSet wants to make a one-phase optimisation (when
there is only a single Action, for example) as it needs to know the number
of Actions, not necessarily who they are.

see above