When ObserveChannels() is called on an observer C which is also a Handler, it
can't tell if those channels satisfy a request it knows about.

* AddRequest() is called on a client C (because it, or something else,
requested a channel).
* C opens a tab or similar to represent the ongoing request.
* The channel appears. MC calls ObserveChannels() on all matching clients,
including C.
* C doesn't know that the new channel relates to the ongoing request; it opens
a new UI element for it.
* Now HandleChannels() is called, including the channel and the requests it
satisfies. Only now can C tell that the channel referred to that request.

The obvious fix is to add ao: Requests_Satisfied to ObserveChannels(), just as
the related ChannelDispatchOperation is already a parameter to
ObserveChannels().